13D Filing: Adams Street Partners LLC and Rimini Street Inc. (RMNI)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

Rimini Street
Inc.

(Name of Issuer)

Common Stock, $0.0001 par value

(Title of Class of Securities)

76674Q107

(CUSIP Number)

Robin Murray

c/o Adams Street Partners

One North Wacker Drive, Suite 2300

Chicago, Illinois 60606

(312) 553-7890

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

October 10, 2017

(Date of Event Which Requires Filing of This Statement)

If the filing person has
previously filed a statement on Schedule 13G to report the acquisition which is the subject of this Schedule 13D, and is filing this schedule because of §§ 240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

NOTE: Schedules filed in paper format shall include a
signed original and five copies of the schedule, including all exhibits. See § 13d-7(b) for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be
deemed to be filed for the purpose of Section 18 of the Securities Exchange Act of 1934 (Act) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act
(however, see the Notes).

This Schedule 13D relates to the common stock, par value $0.0001 per share, in
Rimini Street Inc., a Delaware corporation (Rimini or Issuer). The address of the principal executive offices of Rimini is 3993 Howard Hughes Parkway, Suite 500, Las Vegas, Nevada.

(a) This Schedule 13D is being jointly filed by Adams Street Partners, LLC,
Adams Street 2007 Direct Fund, L.P., Adams Street 2008 Direct Fund, L.P., Adams Street 2009 Direct Fund, L.P., Adams Street 2013 Direct Fund LP, Adams Street 2014 Direct Fund LP, Adams Street 2015 Direct Venture/Growth Fund LP, Adams Street 2016
Direct Venture/Growth Fund LP and Adams Street Venture/Growth Fund VI LP (collectively, the Reporting Persons). Adams Street Partners, LLC is the managing member of the general partner or the managing member of the general partner of the
general partner of each of these entities and may be deemed to beneficially own the shares held by them.

(b) The address of the principal offices of each
of the filing entities is One North Wacker Drive, Suite 2300, Chicago, Illinois, 60606.

(c) The principal occupation of each of the persons set for on
Schedule I hereto is the venture capital and growth equity investment business. The information set forth in Schedule I hereto is incorporated herein by reference.

(d) None of the Reporting Persons have, and to the best of each Reporting Persons knowledge, during the last five years, each of the Reporting Persons
has not, been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e) None of the Reporting Persons have, and to
the best of each Reporting Persons knowledge, during the last five years, each of the Reporting Persons has not, been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction resulting in a judgment, decree
or final order enjoining future violations of, or prohibiting or mandating activity subject to, federal or state securities laws or finding any violations with respect to such laws.

(f) Each of the Reporting Person is a United States citizen.

On October 10, 2017, GP
Investments Acquisition Corp., a Cayman Islands exempted company (GPIA), deregistered as an exempted company in the Cayman Islands and domesticated as a corporation incorporated under the laws of the State of Delaware. Also on
October 10, 2017, Lets Go Acquisition Corp., a wholly-owned subsidiary of GPIA (Lets Go), merged with and into Rimini Street, Inc. (RSI), a corporation incorporated in Nevada in September 2005, with RSI
surviving the merger (the first merger), with the surviving corporation then merging with and into GPIA, with GPIA surviving the merger (the second merger and, together with the first merger, the
mergers). Immediately after consummation of the second merger, GPIA was renamed Rimini Street Inc. (RMNI) and as of the open of trading on October 11, 2017, the common stock, warrants and units of
RMNI began trading on the NASDAQ Capital Market as RMNI, RMNIW and RMNIU, respectively.

On May 16, 2017, GPIA,
Lets Go, RSI, and, solely in his capacity as the initial Holder Representative (as defined in the Merger Agreement) for the limited purposes set forth therein, the person specified as such in the Merger Agreement (the Holder
Representative) entered into an Agreement and Plan of Merger (as amended, the Merger Agreement), as amended by Amendment No. 1 thereto, dated June 30, 2017 (Amendment No. 1).

On October 10, 2017, pursuant to the Merger Agreement, and following the domestication of GPIA as a corporation incorporated in the State of Delaware on
even date, Lets Go merged with and into RSI, with RSI as the surviving corporation. The surviving corporation from the first merger subsequently merged with and into GPIA on October 10, 2017, with GPIA being the surviving corporation and
renamed Rimini Street Inc. immediately after consummation of the second merger. On the effective date of the domestication, each issued and outstanding ordinary share, par value $0.0001 per share,

of GPIA prior to its domestication converted automatically by operation of law, on a one-for-one basis, into shares
of common stock, par value $0.0001 per share (the Company Shares), of the Company after its domestication as a corporation incorporated in the State of Delaware, as renamed Rimini Street Inc. In connection with the mergers,
all shares of Series B Preferred Stock, Series C Preferred Stock and common stock warrants of RSI held by the Reporting Persons were converted into shares of common stock of RMNI pursuant to the terms of the Merger Agreement.

In connection with the RSIs issuance of Series B Preferred Stock in June 2009, the Reporting Persons acquired 2,569,704 shares of Series B Preferred
Stock at a purchase price of $3.8915 per share.

In connection with RSIs issuance of Series C Preferred Stock in October 2016, the Reporting Persons
acquired 56,441,036 shares of Series C Preferred Stock at a purchase price of $0.1772 per share.

In October 2017 the Reporting Persons acquired 497,018
shares of the common stock of GPIA in an open market purchase at a price of $10.06 per share.

The Reporting Persons hold their securities of the Issuer for investment
purposes. Depending on the factors discussed herein, the Reporting Persons may, from time to time, acquire additional shares of Common Stock and/or retain and/or sell all or a portion of the shares of Common Stock held by the Reporting Persons in
the open market or in privately negotiated transactions, and/or may distribute the Common Stock held by the Reporting Persons to their respective members or limited partners. Any actions the Reporting Persons might undertake will be dependent upon
the Reporting Persons review of numerous factors, including, among other things, the price levels of the Common Stock, general market and economic conditions, ongoing evaluation of the Issuers business, financial condition, operations
and prospects, the relative attractiveness of alternative business and investment opportunities, and other future developments. Except as set forth above, the Reporting Persons have no present plans or intentions which would result in or relate to
any of the transactions described in subparagraphs (a) through (j) of Item 4 of Schedule 13D.

(a,b) Regarding aggregate beneficial ownership,
see Row 11 of the cover page of each Reporting Person. Regarding percentage beneficial ownership, see Row 13 of the cover page of each Reporting Person. Regarding sole power to vote shares, see Row 7 of the cover page of each Reporting Person.
Regarding shared power to vote shares, see Row 8 of the cover page of each Reporting Person. Regarding sole power to dispose of shares, see Row 9 of the cover page of each Reporting Person. Regarding shared power to dispose of shares, see Row 10 of
the cover page of each Reporting Person. The percentage listed in Row 13 for each Reporting Person was calculated based upon 58,580,796 shares of Common Stock outstanding as of October 17, 2017.

(c) Except as set forth in Item 3 above, the Reporting Persons have not effected any transaction in the Common Stock of the Issuer during the
past 60 days.

(d) Under certain circumstances set forth in the limited partnership agreements of the Reporting Persons, the general
partner and limited partners of the reporting persons may be deemed to have the right to receive dividends from, or the proceeds from, the sale of shares of the Issuer owned by such entities of which they are a partner.

(e) Not applicable.

The Reporting Persons intend to enter into an agreement with the Issuer to grant the Reporting Persons rights of first refusal to purchase
their pro rata portion of any new securities that may be issued by the Company for a term of five years.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: October 22, 2017

SCHEDULE I