I don't recall anyone posting about using it with the 6p, but it should. I know several people have posted about using it with the S7, which is running Marshmallow, so I'm guessing you'd be fine.
I use the gear s2 with a Nexus 6p. Almost everything works. Native Google messenger has no replying capability but notifications come just fine. Hangout works fine and you can reply if you install Android wear. Google now cards are a hit and miss, not all are visible. Gmail works flawlessly. I like the watch for what I paid but Samsung still needs to put a lot more effort to make this device successful.