Gassiev got reinjured and apparently is now returning to fitness and likely to be ready by around March time. No idea why Miller vs Gassiev will happen in the OP's mind one is with Top Rank and one with Matchroom. Co-promotions only seem to happen if a belts on the line. I can see Gassiev having a couple of warm ups. One against a no name like Devin Vargas. Then one against someone like Dawejko before he steps up. But my guess is that by this time Usyk vs AJ would have happened and if Usyk loses that they will do the Gassiev rematch at Heavy
If Gassiev is back to his best then I think he beats Miller. Miller looks invincible against slow, sloppy fighters that he can smother and grind down, but Gassiev would be moving around with his high guard exploding one or two tight punches down the pipe and racking up points. I don't think he gets the KO, but I think he wins on points in a similar fashion to Adamek vs Arreola.
There are a lot of things to suggest that. Gassiev has much better technique, balance, he knows how to fight off the back Foot and counterpunch. And he has better 1 punch power than both Duhaupas and Washington, both of whom were able to stop Miller on his tracks on spots with their punches.