If guy goes 7th gate he'll destroy Kakashi, let's all take a step back and look at Kakashi's stamina/chakra:
-uses kamui, there goes a huge portion of his chakra. (I'm also sure guy could dodge it)
- Chidori, same thing as above basically, but not as energy-depleting.
- Guy's stamina will keep increasing as he opens gates, while Kakashi's will wear down.
Plus, hirudora is insane.