First create a local overlay list:
echo '<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE repositories SYSTEM "/dtd/repositories.dtd">
<repositories xmlns="" version="1.0">
<repo quality="experimental" status="unofficial">
<name>bingch</name>
<description>bingch local ebuilds</description>
<homepage>https://gitlab.com/bingch/gentoo-overlay</homepage>
<owner type="project">
<email>bingquick@hotmail.com</email>
<name>Bing Chen</name>
</owner>
<source type="git">https://gitlab.com/bingch/gentoo-overlay.git</source>
<feed>https://gitlab.com/feeds/bingch/commits/gentoo-overlay/master</feed>
</repo>
</repositories>' > /etc/layman/overlays/bingch.xml
Then make it known to layman and add it to your local overlays:
layman -L
layman -a bingch