https://github.com/hippie68/gogcheck

Use this to check GOG installers before installation. Should still be accurate and active most recently updated 3 months ago.

Use FitGirl or SteamRip. I also use SteamUnlocked which people say are unsafe but I've downloaded over 100 games from there and had no issues, so I won't recommend it but just adding my experience.

Also make backups to your PC using macrium reflect or other backup programs so you can restore from it in case you get viruses.

I thought the only site you should ever trust is the original one, GOG.com..

I've never had an issue with anything from that site. Did you open up task manager to see if maybe your hardware is struggling somewhere? Maybe there's some unwanted process crypto mining

i have 1TB worth of GOG games, and i got all of it from there. Never experienced any malware.