[gentoo-desktop] Put program in menu

[J.A.H.]

Why when I emerge an x-windows package it only sometimes installs itself in the gnome menu system (depending upon the program). Other times I have to manually  put the program in in addition to manually selecting an icon to go with it in the gnome menu.  Some of my installed x-windows programs are not in my gnome menu so for that reason I do not know they are installed after I install them. What is the recommended (gentoo) procedure for setting up the little program launchers in the gnome menu???