Assuming a fix isn't in, I think Usyk edges it. Both guys have great stamina and are good combination punchers, but I think Usyk is more mobile whlie Gassiev can be a bit slow on foot. Actually, I think Usyk's hand speed is better too. Both are experienced against good competition but I think Usyk has the experience edge. I think Gassiev is actually a bit slicker, but I see Usyk's angle-outs as a good weapon to avoid getting cornered.
As I see it, Usyk will closely win the first half by using his superior hand and foot speed to flurry combos while getting out of the way as Gassiev moves forward. Gassiev won't sustain anything significant: his combination of upper body movement and Usyk's relative lack of power will see Usyk outpointing, but not battering Gassiev. Gassiev will probably start to lay it on in round 7 or 8 and progress with more combination punching, But Usyk will survive and get the nod, maybe suffering a KD along the way. He's a smarter, more versatile and better conditioned fighter than Dorticos, so I think he'll have enough tricks up his sleeve and enough experience to keep his composure and survive.
Not betting on this one though. It's 50-50. Since it's in Russia, it might be Gassiev 120-107 if Usyk knocks him out.