I haven't tried to install but there are several things I noticed.
There are two sets of ebuilds for the same program. "dev-lang/gambas"
which is the version you tried, and "dev-util/gambas" which seems to be
slightly less up to date but from a better class of repositories (Funtoo
is a distro in and of itself).
I'd suggest downloading the most recent ebuild from various repositories
and comparing them One of the older versions might work, and you'd just
have to set up your own local overlay to pull in the latest 3.10.0 version.