In the installation of Gentoo, you'll be much more encouraged to compile your own kernel, an experience that any power user of Linux should go through. :)
Arch by default uses systemd for its initialisation. systemd is growing much more popular, and most distributions are moving over to it in place of the old System-V style init system. Gentoo uses this older init system by default, but is aided by OpenRC. Gentoo however does have systemd available in Portage.
There's one similarity that I'd like to mention though: you can learn a lot about Linux just from installing either distribution!
