In order to install this overlay in Gentoo :

Install layman if you don't have it already:
> emerge app-portage/layman

Synchronize the layman repo list : 
                   
layman -L
                   
You should see my repo been displayed in the list.

Then you need to add your layman overlays to your make.conf:
> nano /etc/make.conf
source /var/lib/layman/make.conf
                                    
To add the overlay : 
                   
layman -a gbin
                   
The packages should be available.                   
                   
To delete it : 
                   
layman -d gbin