http://www.scifijapan.com/articles/2015 ... era-facts/
The section in question:
1. Contrary to popular belief, Gamera has fought Godzilla… just not in a actual movie. Daiei teamed up with Toho Studios for a “Kaiju Show” starring the rival studio’s top mon-stars in a one-act “Godzilla vs. Gamera” event, performed at the Expo’s “Festival Plaza” venue. Comedian Kon Omura (“Cornjob” himself from GAMERA VS. GUIRON) was the MC. Godzilla’s original suit actor, the legendary Haruo Nakajima, was booked for three performances per day, but on account of the complexity of a live “suitmation” event, the show times were scaled back to once a night. So, for ten days in March 1970, Godzilla clashed with Gamera!
Don't know how reliable this is - no sources are cited. It's entirely possible that this is some sort of joke.
Would be intersting to know more about it, if it actually happened.