Not a gmail invention perhaps, but also not per RFC. That some use it to mean something special is not in the RFC. Actually, a significant number of SMTP servers don't even implement the required parts of the related RFCs, let alone fancy things like plus handling.
You're right. Originally the + sign in an email address was an indicator to the Andrew Message System's delivery agent to process the email in an extensible way. The syntax was +<keyword>+<args>. As an example. you could use
"user+dir-insert+misc" to route the message to the "misc" directory in the user's mailbox structure. An unknown keyword would just get ignored and the mail delivered as usual, giving the behavior as used today.