Chagaev was a proven solid heavyweight, whereas Gassiev hasn’t proven to be the goods at heavyweight so far. Struggled against a mid-40s Pulev getting pieced up early on and lost to Wallin. He’s been able to pick up a belt after the heavyweight division became a free-for-all after Usyk relinquished his belts, but I highly doubt Gassiev has improved substantially since the Wallin fight. Chagaev points victory in a clear, but competitive fight. I could see Gassiev having some success stylistically with uppercuts and bodyshots on the inside. Chagaev would have to watch out for those uppercuts and that sickening left hook to the body that Gassiev throws, and Chagaev is vulnerable to those punches, and despite being better than Wallin, he wasn’t as rangy or even as nimble, so Gassiev does have a chance with the tools he has, but I’d still favour Chagaev based on what we’ve seen so far.
Chagaev would dominate Gassiev, two-handed, quick pressure. Chagaev by corner-stoppage, pretty one-sided.