Gas caps today seem to all be constructed using inner and outer subassemblies that are constructed to "release" (click) when tight thus maintaining a fixed pressure in the fuel tank. That mechanism is failure-prone.

I had a similar problem on the fuel cap on my BMW motorcycle. To get the cap off, I carefully drilled a small hole through the outer cap going through the upper side of the inner cap without going completely through. If you then take the drill bit out of the drill, you can reinsert it in the hole effectively pinning the outer and inner assemblies together allowing you to remove the cap. In my case, I put a sheet metal screw into the hole permanently fastening the two parts of the cap and defeating the "clicker" assembly. Still works great after six years.