As a Gentoo user who has used Arch in the past, I have no clue what problems this commenter could have run into because paritioning the drives is exactly the same for both distributions… if they were able to figure it out for Arch, then they can do it for Gentoo
Well, configuring the kernel is where things get tricky and is the major difference between the Gentoo and Arch installation, so that makes sense.
To be annoyingly pendantic, you did get past partitioning the drivea, then!