I imagine you have already sorted this out, but I'll reply anyway for anyone who might be in a similar pickle and is checking out this thread...
You just have to change either Google Drive to another path letter (if it will allow that?) and then revert your USB drive back to the path letter of "G:" – OR – leave Google Drive remain as "G:" and choose a new path letter for your USB drive (choose something further down the alphabet so the next thing you install that automatically grabs a drive path letter will not infringe on your existing path letter assignments), and then you must edit ALL your MC Playlist files (easy enough, just time consuming if you have a LOT of playlists) to the new path letter. As long as you have not changed any directories on the USB drive, then editing will be easy.
0. As always, before editing or making ANY changes to program/system files, BACK UP the files in case they need to be restored.
1. Open a ".m3u" playlist file in Notepad.
2. From the drop menus, select "Edit" and select "Find & Replace" or "Replace"
3. In the dialog box, the first text field (Find) should have the previous path letter, colon, and slash (backslash or forwardslash - whichever applies), and the next text field below it (Replace with) will have the new path letter along with the colon and slash.
Example :
Find: G:/ (or G:\)
Replace with: Z:/ (or Z:\)
4. Hit "Replace" to carefully go one file/line at a time and ensure that no part of any file name and/or directory (folder) is also modified. Or, if you are confident, just hit "Replace All", and let the changes apply automatically through the entire file. NOTE : If a single playlist has hundreds (or even thousands) of files in it, it will take a moment or two to zip through the entire list automatically using "Replace All" (doing this manually one at a time WILL feel like an eternity).
Since I don't know for sure (as I don't use Google Drive in such a way that it would mess with my volume drives), I will only speculate, that if you do elect and manage to change Google Drive's path letter from G to something else (and restore your USB back to G), then you may also have to similarly modify any playlists or shortcuts (anything requiring a path-file target) to any files contained within or linked to Google Drive with the new path letter, which may or may not be as easy as editing the m3U files in MC.
Good luck to all.