We are not aware of any such issue of Gmail.
Is it possible that the mail was corrupted on the way from Gmail?
Please, compare the raw data of the mail on GMail server and on your mail server.
For GMail, go to Sent items, select the problematic mail, in menu select "Show original".
If you send us the two mails (one from Gmail, second from your mail server) we can analyze it. You can send it to firstname.lastname@example.org - please send it in ZIP file to prevent modifications on the way.