gem-man doesn't follow the standard $MANPATH directory structure (man/manX/Y.X where X is the section number and Y is the manual page) but instead uses a single flattened directory (man/Y.X) to keep manual pages from all sections.
For example, observe how man(1) cannot find bar.3, whereas it can find foo.1:
$ mkdir -p man/man1
$ touch man/man1/foo.1
$ touch man/bar.3
$ man -M ./man -a bar
No manual entry for bar
$ man -M ./man -a foo # success!
This prevents us from adding man/ subdirectories in installed gems to $MANPATH and using plain old man(1) thereafter to look up both system and gem manuals.
Thanks for your consideration.
GH-8: support gems that follow $MANPATH structure
I've added a commit to fix this issue. Please review, merge, and release.