Astoria Gas Turbine Power S-4/A Registration of securities issued in business combination transactions (amended)

Exhibit 3.209

State of Delaware

Secretary of State

Division of Corporations

Delivered 01:41 PM 03/21/2013

FILED 01:34 PM 03/21/2013

SRV 130342534 – 5307432 FILE

CERTIFICATE OF FORMATION

OF

NRG TEXAS GREGORY LLC

1.Name: The name of the limited liability company is NRG Texas Gregory LLC.

2.Registered Office: The address of its registered office in the State of Delaware is Corporation Trust Center, 1209 Orange Street, in the City of Wilmington 19801, County of New Castle. The name of its registered agent at such address is The Corporation Trust Company.

3.Organizer: The name and address of the sole organizer of the limited liability company is Lynne Przychodzki, NRG Energy, Inc., 211 Carnegie Center, Princeton, NJ 08540.

IN WITNESS WHEREOF, the undersigned has executed this Certificate of Formation of NRG Texas Gregory LLC this 21st day of March, 2013.

/s/ Lynne Przychodzki

Lynne Przychodzki

Authorized Person