Guy wins this
--him keeping up with juubi jin madara in the 7th gate is a clear indication that MS users such as itachi are not tracking him, and wiith the drawbacks of overusing his MS, just ends up dropping itachis speed and reflexes even more,
--Guy then proceeds to use hirudora to eradicate V3 susanoo,and can very easily use his gated speed to evade everything itachi throws at him, as well as outlast his V4 (which itachi is not using for long periods of time at all), and guy can also use hirudora followed by asakujaku or another hirudora to kill him off
--Tsukuyomi is also not happening when itachi has no way of tracking guy, and the latters fighting style prevents him from falling prey