The Banner for the Gamera Games 5th Anniversary features Leonardo from TMNT even though the game (TMNT Shredder's Revenge) is not featured here, does this mean the game is coming or is this an oversight from using a generic banner used on other storefronts?

gOg is not "copy pasting steam" they are using the assets the publishers have given thme to use

Nothing new, they were using Doom 2016 graphics for Bethesda sale in the past and people were also speculating.

That my be so, still games featured in the banner such as TMNT Shredder's Revenge and Dyson Sphere Program are nowhere to be found.

and that's because the publishers are not wasting monies creating different assets for different stores

It means neither of those two things.

I'm sure GOG isn't going to go out of their way to remove characters from banners even if GOG doesn't sell games with those characters in it.

But to call it an "oversight" makes it sound as if they would never do that if they actively knew that such a thing was happening. Except there isn't necessarily any reason why they wouldn't do that anyway.