Yes, click it a few times. However I suspect that you had a CEL (Check Engine Light) and someone told you that it was the gas cap. It might be, but then it might not. The light does not say gas cap. There is a code that indicates there is a leak in the fuel vent system. That often means someone did not put the gas cap on tight. Cheap to try and hope that was it. However there are other possibilities that the same code may result from. It would be a good start by telling us a little more.
Make, Model, Year and Mileage is your car? Did someone actually see the light and check in a manual as to what it meant? There should have been a code like [P1234] find out what code you had and let us know in a reply to this thread.
[b] The manual says that there might be water in my gas and to use a premium gas.[/b]
Really. How would it guess you might have water in the gas?
Unless your Owner's Manual says you need or should use premium, there is absolutely nothing to gain and it will cost you more. I might be talked into believing your car really runs well after you fill it up, but I would be more inclined to believe it is doing so because there is a problem with the in tank fuel pump and the filter/screen that is in the tank. Premium will never fix that. If it really is the problem then premium gas will do nothing, You will need to have the tank cleaned.
Now if you have a Service Soon light on, then you need to have your car serviced. Maybe oil change, filters etc. You need to see your owner's manual for more specific information.