Yeah it is also possible. I just thought you wanted an easy solution. If you want to do it manually you need to get the SDK installed with the fastboot binary. You can then flash the custom recovery manually from terminal with adb and fastboot.
CyanogenMod has a quite good wiki you can follow here:
http://wiki.cyanogenmod.com
Not sure if there is a guide for the EVO though, but it might answer some of your questions. You can also check
http://forum.xda-developers.com for a guide or something. Gentoo should not hold you back anyways.