It really doesn't matter where you mount it, as long as you use that same path throughout all the process.
Being that said, you should really stick to /mnt/gentoo, because a) it clearly express what's in there, b) will avoid silly errors, c) if you mount gentoo in /mnt when everything else that's mounted under /mnt/whatever will not be reachable.
I really doubt arch mounts anything directly in /mnt. They, just like all linuxes, surely mount stuff in subdirectories under /mnt.
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