Like anything else, the best way to see if you like it is to try it. If possible, start with a VirtualMachine. PCBSD is aimed at giving you a desktop, but the last time I tried it, sharing space on a laptop, it was actually harder to get working than a standard FreeBSD install.
You will find similarities between ports and portage--Daniel Robbins worked on FreeBSD before developing Gentoo, and that experience strongly inspired a lot of Gentoo, at least when it first started.
You'll run into issues--these forums a great resource, and as others have said, the
Handbook can be useful, though it can sometimes be behind the times or make the simple things overly complex. There is also a wiki at
https://wiki.freebsd.org/ which can, at least at times, be more up to date.
Aside from the differences--which will be less to a Gentoo user than some other flavors of Linux--some of the hardware support may lag behind Linux. For example, though some folks have had success, I haven't been able to get
Skype working. A particular wireless card I had didn't work until FreeBSD-10.
However, many of us quite successfully use FreeBSD as a desktop. In the same way that Linux users sometimes have to find workarounds if they're going to avoid Windows or OSX, we sometimes have to dig a little to find workarounds when using FreeBSD as a desktop.
However, often, using a system as a desktop is one of the quicker ways to start finding one's way around the system, and the disadvantages are usually minor.