Yeah I backtracked a little bit from my initial prediction. Volkov is indeed the bigger guy though he was about 250 lbs until his last fight where he came in significantly bigger. And he is very sharp with his straight punches and distance management. I actually do think he has a very good kicking game but he mostly uses front kicks to the body. In this matchup he'd probably be better served with strong legkicks.
It might be a lot closer than I thought. But I look at the Hardy fight and how Volkov was having problems with Hardy's speed. I think that fight would've been close if Hardy didn't break his wrist early on. (Granted, caused by Volkov highkick but still...). If Hardy can look decent against Volkov with nothing but speed and size, I think Gane can shine.
Volkov has been significantly bulking up since the Blaydes fight - in that fight he was under 250 and was getting absolutely rag-dolled. I think he made the mistake in that point of his career of thinking that speed is the most important asset but he forgot for a moment this is Heavyweight (chin/power/skill all more important).
I've never really thought of Volkov as a great kicker, though he definitely throw a solid teep. Definitely can be dangerous when he does throw them but he doesn't set them up really. Gane is actually one of the most impressive HW's when it comes to kicking; can go High/Medium/Low, can teep/oblique/switch, really has all the bag of tricks. It's just kicking at HW is such a dangerous proposition - few have the athleticism to surprise opponents, few have the depth of technique to effectively set them up, and it uses a ton of energy (HW's really only throw power kicks).
I see your logic in the Hardy vs. Volkov match-up, but I also think it's kind of MMA-Math; there were so many crazy variables in that fight to consider: it was supposed to be JDS until a month before the fight then it switched to Hardy + it was the Volkov's first big fight in terms of billing (Co-Main) + it was in Moscow (so imagine the pressure). It felt like Volkov kind of laid an egg, but ultimately I felt like he did the same thing that Gane did against Rozenstruick which was to fight the most conservative fight possible....and Volkov even turned it up late in the 3rd while Gane was content to cruise (to be fair he had seen the Overeem horror-lip in the last second of a 5-round fight, so probably didn't want to take any chances).
Hardy is very fast/explosive for HW, he just doesn't have good long-term endurance and is limited in depth of technique and fighting experience. So was always a very good match for Volkov, he probably just didn't want to shit the bed in a short-notice fight in his home city/country trying to go on a title run. Same thing with Gane vs. Rozenstruick, it makes sense why it was a bad fight - the style match-up favored Gane and he didn't want to take dumb risks against a dangerous counter-striker that is predictable when he is easily winning.
Ultimately every fighter is unique and every fight is as well, so there is such a crazy myriad of variables and circumstances to consider. These two are such experienced, technical fighters that I think the only clear outcome is it won't be an easy fight for either man.