What exactly is the lineage of IGAS ownership? Did G-T reacquire it from Telecast Enterprises either for the 1976 four-show run or for the syndicated run made in Hollywood? Did Fremantle
ultimately wind up owning it?
I can't tell you under whose auspices Goodson produced the 1972 and 1976 versions. "Telecast Enterprises" does not show up in any search of any historical records I can find after 1967.
In 1992, The Carsey-Werner Company announced that they had acquired the rights to the format, and were going to launch a version in the fall of 1993. That version didn't happen, but they made the 1980 version on Oxygen, and they probably had a hand in (or at least signed off on) the 2006 GSN version. The press release for the 2023 pilot says that it's "based on the format owned by Werner Entertainment, Inc." (Carsey and Werner have not been producing partners for a long time.) Fremantle, as best as I can tell, does not have any ownership stake in the format. They just have possession of the old episodes.
IIRC Gil said in his book that Beatrice Foods was involved at some point.
Gil is specifically referring to
What's My Line? in a brief passage where he says, "In 1969, in a manner too involved to detail here, the rights to the program passed to Garry Moore's Redwing Productions. In 1971, Redwing sold all of its assets to a conglomerate. If after reading the following pages anybody gets a hankering to revive What's My Line? on television, he will have to negotiate for the rights with Beatrice Foods, Inc., the people who also bring you Stiffel lamps, Samsonite luggage, and Dannon yogurt."
So take that as you will. This passage might just be about WML? or perhaps Garry's company acquired rights to both shows in that complicated deal.