It's giving me anxiety attack.

"The device or network location you selected does not exist or is not accessible. please choose another destination."

but I have absolutely NOTHING chosen and I cannot modify anything.

I already tried to change the installation path for my other SSD because the first one is full but I don't understand what I can do.

And how I tell windows and gog that disk D went in the trash and that now it is called G.

I was having a bad day but that's the cherry on the cake.

You already have GOG Galaxy installed, so the installer will try to update your existing installation, which no longer exists in your case. You really shouldn't move or remove drives with applications installed on them. If D: is now an available drive letter, you can reassign G: to it. If you've already installed applications to G: they're likely to stop working.
1. Press Windows logo + R to bring up the Run box, type compmgmt.msc and press Enter.
2. Under Storage, click Disk Management.
3. On the right side near the top, right-click drive G: and choose Change Drive Letter and Paths.
4. Click the Change button and select D then press the OK button twice.

If the above isn't possible, I'd manually remove GOG Galaxy from its various installation folders and the registry, then I'd run the latest full installer.