They gave GTA2 away for free officially at some point, which might have reduced a desire for it. That's my only guess.
For any on mac who are looking - it is available for free from Porting Kit (which is a Wine-like platform). I am assuming they can install it for free since they're some sort of side project of GOG and I presume they have the original binary from when it was released for free
So presumably anyone else with a Win system could get it from GOG (actually looks like you can get it here - https://gta.com.ua/rockstargames-classics-free-download.phtm... )