I can think of no Steeler ILB or MLB of note who tipped the scales at more than 240 lbs., give or take.
The key characteristic you look for in an MLB/ILB is read-and-react, or instincts if you prefer that term. Players don't pop out of the womb with it. That ability is honed over years. While you might look at Gary's size/strength/speed combination and see a guy who can play the position, you'd suffer a lot of pain for a long time from all those wrong first steps. We've already seen him misread a play doing something as simple as a short drop and end up chasing down the field.
If all a guy has done is play the line or the edge, his knowledge of the field is what's in front of him. Experience in coverage concepts and developing "feel" in a position that demands 360 degree awareness is limited or entirely foreign. You can even look at some top college prospects and wonder how they'll transition. With some of those guys you can go through an entire game tape and not see them straying far from the line of scrimmage. And it's hard enough for college ILB/MLB transitioning from a bunch of read-option/spread/RPO/whatnot to defending pro offenses with greater speed and sophistication.
In short, no, never.