Far as I know, it shouldn’t matter if it was a gift. If you own the game, then you own the game. Neither Funcom nor Steam care who actually paid for it, as long as it was paid for and is a legal copy. If the game shows in the Library of your Steam account (i.e. not on family share), then you should have full rights to reinstall the game as many times as you need to.
If you’re going to try this option, may I recommend doing a full wipe as recommended in the sticky post in PC Bugs/Updates forum.. Basically, all this means is to delete your entire Conan Exiles folder before reinstalling. Good luck. You’re one of the ones who haven’t been able to run the game for quite a while now.