The solution is here :
https://superuser.com/questions/1663797/google-drive-files-not-appearing-in-spotlight-and-alfred-after-update
You have to manually enable Spotlight indexing again. Open a Terminal window and type:
sudo mdutil -i on /Volumes/GoogleDrive*
Worked for me on MAC OS Big Sur 11.5.2. Just wait 5 min and the files should show up. Of course, you have to add /Volumes/GoogleDrive/MyDrive to your search scope first.