What is the type of external sharing link?
You could go to Microsoft 365 admin center -> Users -> Guest users -> Add a guest user -> Invite user -> Add the Gmail user.
Then re-share the file with the Gmail user.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.