Yeah, gap buffers LONG predate RMS.
The author of TECO, Dan Murphy, wrote an article about its history for the IEEE Annals of the History of Computing. A PDF copy of the article is available here: https://opost.com/tenex/anhc-31-4-anec.pdf
Alas, it does not discuss the gap buffer.
The book The Craft of Text Editing claims the gap buffer technique was first used by TECO (http://www.finseth.com/craft/#c6.6), although this claim is given no support.
Emacs was initially just a bunch of macros (in Eugene Ciccarelli's TECO init file IIRC) that provided an easier interface to TECO once a realtime display mode (^R mode of course!) had been added.
Yes, in TECO, control R was a keyword in the language. If you don't know TECO it looks like line noise. For controlling an editor, that's not necessarily insane.