The (exceptional) doco is there to guide you
Originally Posted by handbook
Make sure you use your kernel image filename and, if appropriate, your initrd image filename.
That seems pretty unequivocal.
You have makeactive in the wrong place, but it's only relevant to the Windoze loader (it sets the boot flag in the partition table). You could remove it altogether.
Chainloader is a command to grub to transfer control to another loader (ntldr in XP) and quietly disappear.
Grub (and its commands) is documented at gnu.org
BTW there is now a gentoo specific forum here at LQ.