Also because of their no-DRM policy, some Windows games from GOG are actually more compatible with Wine because the DRM was removed.

In my Steam collection I actually have a situation of a game which won't run in Linux from Steam no matter what I do, yet the pirate version of that game runs just fine in Linux (and in fact that's the only way I can enjoy my purchase), whilst in my much larger GOG collection I have no such situation.

Heroic Launcher connects with GoG, so you don't need to wait for this. I'm still happy it's coming, but we do have other options.

I also bought those games when they were on sale!

Personally, I like the Ultima series of games

I started with Bottles, then switched to Lutris, which dramatically improves the experience. I now (and for a while now) use Heroic Launcher. It's my preference. Lutris has still been useful, in particular for installing Escape from Tarkov from an install script for Singleplayer Tarkov (or whatever it's called now). Heroic doesn't support that as far as I'm aware. It does connect with Steam, GoG, and Epic though, so you can buy and play games from those through it.

Yeah I usually do the same. Offline installers are a pretty big benefit. I like to know a lot of games are available offline.

Lutris & Heroic already install all windows GOG games into Wine +/- Proton prefixes using some simple automation. The bar for an official GOG launcher to achieve the same isn't that high if they want to go down that route. Proton is freely available and open source, so GOG can use it too if they want (and would be sensible to do so).

All of those old games just run through wine or dosbox. They install just fine through GoG Galaxy. GoG Galaxy has been on macOS for years.

Proton is not part of Steam. It's a package that you can run games on from anywhere. Yes, this will need WINE and likely Proton, and it's easy enough to have the launcher use those, as Heroic and Lutris already do (or as you can do from the terminal).