It refers to your main refinery/warehouse.
Therefore, the gas refinery expansion allow you to store in your main refinery the gas cubes drilled from those towers that usually send the production straight to the local market.
The advanced tower Gas Refinery Expansion states "Also, if you want, you can store in the refinery the gas barrels..."
Is "the refiner" the Gas Refinery Expansion or your main warehouse? I ask because there are no spots to hold gas cubes. Do I just place them on the tile or somewhere else?