You didn’t describe what model or year Subaru you are driving, but how to open a Subaru gas tank has been pretty standard over the years.
Look down at your feet, and locate the gas tank door release lever, which should be on the driver’s side floor, on the left-hand side. The lever has a gas pump icon on it. Pressing this lever will make the door to the gas tank (which is likely on the passenger’s side of the car) pop open.
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