The ebuild says GPL-3+, but I cannot find the "or any later version" clause anywhere, so I think we have to assume GPL-3, no?
There is a LICENSE file, but I cannot find any license notice (neither in README nor in source file). So we don't know what the author's intentions are. I suggest that you ask upstream for clarification. For example, they could copy the standard GPL notice (as in https://gitweb.gentoo.org/repo/gentoo.git/tree/licenses/GPL-3#n634) into their README.md or (even better) into each source file.
I have clarified with upstream before adding it, but he hasn't made the changes to his repo yet. We are okay here.