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

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ITEM 1. Security and Issuer.

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.

ITEM 2. Identity and Background.

(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.

ITEM 3. Source and Amount of Funds or Other Consideration.

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,

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