I don’t think you can lump Endeavour and Garuda together. Yes, they’re both based on Arch but Endeavours basically is Arch with a GUI installer and sane defaults while Garuda changes a ton of things and adds a ton of customisations that make it very different from a plain Arch (or Endeavour) system.
Usually Steam. I like the idea of GoG, but a lot of the time if you want mods you’re basically forced to buy it on Steam because of the Workshop. Also, I kinda like having everything in one place.