If people are looking for missing 'git gui' or 'gitk' commando, they'll have a hard time figuring out how to get them. Is there anything else apart from these two commands that get enabled by 'tk'? It definitely needs a local USE flag description.
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=55fe669761e175492072bf6caec1bcdc05f6c33b