As someone who games solely on a mac, I can say that I disagree with you.

It's generally common sense to work out which games work natively on your OS and which games are wine ports (hint: older games are almost certainly not native to your OS).

I'm a Linux user. I don't agree with you.

GOG lists which games use wine and which ones don't. That's more than some other verndors selling wine ports do.

GOG is also providing support to those wine ports.

Effectively, the game is sold as a Linux game because it can be played on Linux, and if there are problems getting it to play on Linux, GOG will help.