I realize I’m going to be a bit of an odd case here.
I’ve got a pretty decent library on GOG, and have most of it backed up on archival dvds. I have limited space on my pc ( and in general ) so I have to reinstall games frequently. Problem is - I rarely, if ever have decent internet because I live on a boat. So I have to use windows at sea - which I am loathe to do. I have all my legit GOG install files - is it possible to modify GOG installers to never go online?
Well… For that to be the case, you’ll have to set up cache path in your preferences and place install files there (typical location would be $CACHE_PATH/$GAME_ID/gog). Then, you need XML checksum files for each of them (no idea how to get them without running at least first stage of Lutris installer…). You also need all utility downloads in the install script (like unpacker, or extra libraries), placed somewhere nearby (depending on script), and all of the Wine versions & Winetricks cache files needed for Windows games as well.
Alternatively, you can download the Lutris install scripts (which I believe you’ll do either way), and edit out gogid and gogslug values. This will cause Lutris installer to request for the location of pre-downloaded installer instead of trying to get it from GOG site. That being the case, it does not preclude the need for preparing all other dependencies I mentioned (utility downloads, Wine versions & Winetricks cache files), so you still have to ensure your installers have all dependencies, per installer.
So, if you have enough time to prepare, my advice would be to go the first way (actually both will work, but the first one might be less troublesome in the end – you just need to use symlinks to mounted disk instead of actual installer files) and install/uninstall each of the games once – that should ensure all dependencies are downloaded and ready for use. You’ll have to check if it stores the untility downloads correctly though; I vaguely recall Lutris not keeping them when I tried it out originally (or maybe I remember it wrong, but it’s still worth checking).
BTW, if one of the games happens to be Unreal Gold, you’ll need to run two installers to get a playable install (for whatever reason support for modern drivers was put in a separate Lutris installer).