Timboli: Generally I don't double dip ... as in buy from Steam, then a little later buy from GOG. Though you could say I have done that with the many games I have on disc, that I bought again here at GOG ... many years between though.
StingingVelvet: I double dip like an idiot, but only when the GOG version is insanely cheap. Quake was like $2-3 here multiple times I believe. Not defending anything vis-a-vis this enhanced version, just speaking to the topic of re-buying games here (which I think a decent chunk of people do, hence games that have been $5 on Steam a thousand times coming here and being at the top of the bestseller chart for a week).
Well like I said, I have done it with many games I had on disc.
I've also bought at GOG when cheap enough, if I got the game earlier for free at Epic ... depends on the game though and how much I wanted it in the first place.
I've also re-bought several Tomb Raider games at GOG, after buying them very cheap at Steam ... and for a few it was a triple dip, because I also had them on disc.
There are still a good number of games I got free at Epic, that I would buy at GOG if cheap enough. One such is The Witness, which because it is also DRM-Free at Epic, needs to be a couple of dollars cheaper than it has been at GOG.
Any game I have gotten at ZOOM Platform or Itch.io or even Humble or the freebies at IndieGala, I won't buy again at GOG, as they were all DRM-Free and available as installers.
I can now create my own installers for DRM-Free Steam and Epic games, but I'd rather not if I don't have to. Partly because I want to support GOG, and because it takes time etc to compress to an installer, but also you just never know about missing dependencies. Getting a GOG version is usually a no-brainer.
P.S. I suspect Quake Enhanced will eventually come here, just not until they have milked it enough elsewhere, where they get money from sales of it.