When you say "GOG is installed on my c drive, I assume you're using Galaxy?

I don't use it, so I don't know this for sure, but I believe I saw comments of Galaxy users in the past, where they confirmed this to be possible (= there should be a possibility to change the installation path?)

What I can say with 100% certainty is that you can install your offline backup installers anywhere you want.

Each time, you install a new game, there will a menu pop up, where you can set the installation path for said game - in theory you could install every single game on a different HDD/SSD.