Elton is a hell of a piano player, but that wouldn't have been the kind of music for him. Reportedly, he also auditioned as their singer when they were on the verge of becoming Gentle Giant but fell through.
Tull is a lot less organic than Gentle Giant who retained that quality no matter how cerebral their music was. Tull sounds often angular to me, Gentle Giant just complex, but not angular. A bit like Zappa, his music was demanding, but never angular.
I have this lavish, limited box set from them that sold so surprisingly well, they did two print editions of it due to public demand, and which now fetches more than thousand bucks on the collectors' market.
There was a handful of Gentle Giant fans at my school, they were sort of the Proggers of the Proggers, the elite of the elite. I first got to know them via their Playing The Fool Live album in 1977. It made the lower rungs of the Top 100 of the Billboard Charts, not bad for music so idiosyncratic that it probably only got played on college radio stations.