...Frank Gifford met Kathie Lee Johnson when both subbed on "Good Morning America." CBS had Vin Scully do NFL Football in the '70s, and assigned both Pat Summerall and Scully to golf and tennis coverage during the same stretch. I recall Summerall getting into an on-air argument with an Australian tennis pro doing colour commentary when he repeatedly referred to American football as "your sport"...
I don't remember Gifford filling in for David Hartman on "GMA"; I've always assumed that Steve Bell would go over and fill in instead. I do remember Al Michaels filling in once for Charles Gibson in the late 80s, and he seemed to be pretty good at it.