Unproven ? He won the IBF and WBA belts at Cruiserweight and has one loss on his record and that was in an Undisputed Cruiserweight fight with WBC and WBO belt holder Usyk. I remember people saying Usyk was unproven when he also moved up to Heavyweight but we all know how that turned out .
AJ seems used up, like a 1976 AMC Gremlin. Before Ruiz, he would have won but now probably not. Yes Gassy has sick power but he also has T Rex arms. Shoulder to fingertip can't be more than 22 inches. He's also slow as molasses. But AJ has that chin....
Gassiev is a unknown entity at Heavyweight. He couls be a killer, never seen him hurt, always carries massive power.
Joshua would run like he did in Ruiz II. It would be up to Murat to catch him and if he did lights out for Joshua.
Gassiev being able to put Joshua to sleep is beyond question, but it may be worth considering that Joshua may be just as capable of putting him to sleep. Look at the guys that AJ hasn't either hurt badly, dropped or stopped: you have Usyk, who is a full couple of skill levels above Gassiev as seen when they fought h2h, and has an iron chin to go along with his elite defensive skills, and then you have a couple of guys who are themselves quite skillful and far bigger than Gassiev, in Ruiz & Parker. In his campaign at HW so far Gassiev is 5-0, but against dudes who don't have one tenth of Joshua's pop. I'd be inclined to lean towards Gassiev winning if he can land something flush on Joshua's chin, and would be rooting for that outcome, but even his most ardent haters must admit that AJ is dangerous, and truthfully we haven't seen evidence of how Gassiev takes a proper shot from a ranked guy above the cruiser limit. So there is the possibility of a stoppage in either direction. What's very unlikely here is 12 rounds of 10-9's.