Hall was sucked into the gravitonium, which was packed up and sent away to a S.H.I.E.L.D. facility with him still stuck inside.
Jed Whedon’s hint that we’ll soon get another look at gravitonium may mean that we’ll finally learn what happened to Franklin Hall.
Agents of S.H.I.E.L.D. first introduced the element gravitonium way back in the third episode of Season 1, when Coulson and his merry band of agents attempted to rescue the brilliant scientist Dr. Franklin Hall from kidnappers.
Hall was trying to overload a gravity-distorting device powered by gravitonium before it could be used as a weapon; unfortunately, overloading the device would have destroyed an entire building full of people, so Coulson stepped in and destroyed it.