Is there any way in Gmail to search for invitations (calendar invites) I have received where I have not responded / RSVP'd?
Not with Gmail search. I don't see anything in a message for a calendar invitation that's been responded to that's different one from that hasn't.
The RSVP "Quick Action" in the subject line will show your response, but only when you click on it, and that's separate from the mail message and things you could search for.
This certainly seems like it would be useful. Let's hope Google adds something to find pending invitations soon.