Tldr: neither of those for pure gaming, get the PC37X with a nice sound card, maybe Soundblaster Zx, and you're done for about $250-300. Otherwise you'll want an amp/dac and I couldn't find a better combo until I spent about $2000 (HD 800 S, Valhalla 2, Modi Multibit).
My Sennheiser HD 650s had a pretty good sound stage and audio quality in game, but honestly I felt Senns gaming headset had much better accuracy in producing game sounds (gunshots/explosions/footsteps) and were only outshined when listening to music and the midrange was sweet. So if you want to game and be able to listen to sweet music then do the 6XX. Otherwise the PC37X gaming headset is also on massdrop if you're interested, it's cheaper, and better for straight gaming IMO. It uses the same driver as the HD 598 headphones but is tuned for gaming and adds a decentish mic. It's also labeled as open back but it's not as open as most open backs, it's more of a hybrid and I think it's part of why it's taken me a long time to find a better pair of cans.
I ended up actually returning my HD 650's for Sennheiser HD 800 S's which are EPICCC when it comes to sound stage. I feel like a God in Siege listening to people walk through the buildings or like I'm in the middle of a concert when listening to music. Also can't recommend Schiit audio enough for amp/dac if you go that route, Valhalla 2 and Modi Multibit are a pretty epic combo with the 6XX or the 800 S, especially for classic rock and acoustic.
Also, never tested the K7XX, that one seems to be getting recommended a lot though for gaming focus. But I didn't find a setup better than my PC37X (I had the actual Senn version, but appears the same) with a Soundblaster Zx until I spent about $2000... My original setup was like... $250... You don't need that kind of hardware for ONLY gaming, it's more for listening to music, so you can hear every single instrument perfectly neutral with .000001% distortion and have Nirvanas acoustic album caress your cajones through your eardrums.