Unfortunately this did not work.
xattr -dr com.apple.quarantine "/Applications/Oh My Git\!.app"
When I try to start the Mac app on macOS 12.0.1 "Monterey" I receive the message
“Oh My Git!” cannot be opened because the developer cannot be verified.
It seems to be related to Mac Gatekeeper and app notarisation/signing. Maybe this app could get notarised by Apple? Apple also might waive their fees for open-source, educational apps.