Game is Serious Sam the 1st Encounter. I try to download with browser multiple times and each time it was currupted. I think tried the gog downloader but that doesn't work as it asks me for my email and password but if I type it in it just asks again a few seconds later and does this over and over and over and over and over and over again. What sould I do?

Check if your firewall isn't fucking with GOG downloader.

I once had problem with files being downloaded corrupted, it would happen because my internet wasn't stable enough and sometimes shut down for a few seconds and screw up the whole download. Maybe that's your problem too?