I have some games from GOG that I didn't install using their default folder names because I have my own naming schemes. Unfortunately, this had the side effect of causing Galaxy to be unable to find them.
If possible, I'd like to remedy that without redownloading, hence the question in the topic title.
It is technically possible to do what you want without re-downloading completely, but you may run into issues and it's not supported...
Sorry I should have read that completely, I read that as you installed them in different locations. So you changed the names? It's still possible to do what you want but I assume these games are installed via standalone installers? GOG recommends a re-download via Galaxy rather than using standalone installers.
Even GOG messed this up themselves by naming some games differently in Galaxy then how the standalone installer does. If you post the games I *might* be able to tell you what Galaxy calls them if I own them.