WAIVER UNDER CREDIT AGREEMENT a

Exhibit 10.31

WAIVER UNDER CREDIT AGREEMENT

among

GASTAR EXPLORATION USA, INC.

THE GUARANTORS SIGNATORY HERETO

THE LENDERS SIGNATORY HERETO

and

AMEGY BANK NATIONAL ASSOCIATION,

as Administrative Agent

Effective

March 12, 2009

TABLE OF CONTENTS

i

WAIVER UNDER CREDIT AGREEMENT

This WAIVER UNDER CREDIT AGREEMENT (this “ Waiver ”) executed effective as of March 12, 2009 (the “ Effective Date ”) is by and among GASTAR EXPLORATION USA, INC., a Michigan corporation (the “ Borrower ”), GASTAR EXPLORATION LTD., an Alberta, Canada corporation (the “ Parent ”), GASTAR EXPLORATION NEW SOUTH WALES, INC., a Michigan corporation (“ Gastar New South Wales ”), GASTAR EXPLORATION VICTORIA, INC., a Michigan corporation (“ Gastar Victoria ”), GASTAR EXPLORATION TEXAS, INC., a Michigan corporation (“ Gastar Texas Inc ”), GASTAR EXPLORATION TEXAS, LP, a Delaware limited partnership (“ Gastar Texas LP ”), and GASTAR EXPLORATION TEXAS LLC, a Delaware limited liability company (“ Gastar Texas LLC ”, and the Parent, Gastar New South Wales, Gastar Victoria, Gastar Texas Inc., Gastar Texas LP and Gastar Texas LLC, collectively, the “ Initial Guarantors ”), the lenders party to that certain Credit Agreement dated effective November 29, 2007 by and among the Borrower, the Initial Guarantors, the lenders party thereto or bound thereby from time to time (the “ Lenders ”), and Amegy Bank National Association, a national banking association, as administrative agent for the Lenders, letter of credit issuer and collateral agent for the Lenders and certain other parties (as amended to the Effective Date, the “ Credit Agreement ”), and AMEGY BANK NATIONAL ASSOCIATION, a national banking association, as administrative agent for the Lenders (in such capacity, the “ Agent ”).

W I T N E S S E T H :

WHEREAS, the Borrower, the Initial Guarantors, the Lenders and the Agent are parties to the Credit Agreement; and

WHEREAS, the Borrower and the Initial Guarantors have requested that the Agent and the Lenders waive any default or right to exercise any remedy as a result of the inability of the Borrower and the Initial Guarantors to deliver to the Agent an unqualified opinion with respect to the consolidated Financial Statements of the Parent and its consolidated Subsidiaries at December 31, 2008 and for the year then ended, as required by Section 5.3 of the Credit Agreement, solely as a result of the pending maturities of the Obligations and the Parent’s convertible senior debentures due November 20, 2009;

NOW, THEREFORE, in consideration of the premises and the mutual covenants and agreements contained in the Credit Agreement and herein, the parties hereto agree as follows:

ARTICLE I

DEFINITIONS AND INTERPRETATION

1.1 Terms Defined Above . As used in this Waiver Under Credit Agreement, each of the terms “ Agent ,” “ Borrower ,” “ Credit Agreement ,” “ Effective Date ,” “ Gastar New South Wales ,” “ Gastar Texas Inc ,” “ Gastar Texas LLC ,” “ Gastar Texas LP ,” “ Gastar Victoria ,” “ Initial Guarantors ,” Lenders ,” “ Parent ” and “ Waiver ” shall have the meaning assigned to such term hereinabove.

1.2 Terms Defined in Credit Agreement . Each term defined in the Credit Agreement and used herein without definition shall have the meaning assigned to such term in the Credit Agreement, unless herein expressly provided to the contrary.

1.3 References . References in this Waiver to Exhibit, Article or Section numbers shall be to Exhibits, Articles or Sections of this Waiver, unless expressly stated to the contrary. References in this Waiver to “hereby,” “herein,” “hereinafter,” “hereinabove,” “hereinbelow,” “hereof,” “hereunder” and words of similar import shall be to this Waiver in its entirety and not only to the particular Schedule, Exhibit, Article, or Section in which such reference appears. Specific enumeration herein shall not exclude the general and, in such regard, the terms “includes” and “including” used herein shall mean “includes, without limitation,” or “including, without limitation,” as the case may be, where appropriate. Except as otherwise indicated, references in this Waiver to statutes, sections, or regulations are to be construed as including all statutory or regulatory provisions consolidating, amending, replacing, succeeding, or supplementing the statute, section, or regulation referred to. References in this Waiver to “writing” include printing, typing, lithography, facsimile reproduction, and other means of reproducing words in a tangible visible form. References in this Waiver to amendments and other contractual instruments shall be deemed to include all exhibits and appendices attached thereto and all subsequent amendments and other modifications to such instruments, but only to the extent such amendments and other modifications are not prohibited by the terms of the Credit Agreement or this Waiver. References in this Waiver to Persons include their respective successors and permitted assigns.

1.4 Articles and Sections . This Waiver, for convenience only, has been divided into Articles and Sections; and it is understood that the rights and other legal relations of the parties hereto shall be determined from this instrument as an entirety and without regard to the aforesaid division into Articles and Sections and without regard to headings prefixed to such Articles or Sections.

1.5 Number and Gender . Whenever the context requires, reference herein made to the single number shall be understood to include the plural; and likewise, the plural shall be understood to include the singular. Definitions of terms defined in the singular or plural shall be equally applicable to the plural