This isn’t strictly an answer to your question about installing .gem packages, but you can specify all kinds of locations on a gem-by-gem basis by editing your Gemfile.
Specifying a :path attribute will install the gem from that path on your local machine.
gem "foreman", path: "/Users/pje/my_foreman_fork"
Alternately, specifying a :git attribute will install the gem from a remote git repository.
gem "foreman", git: "git://github.com/pje/foreman.git"
# ...or at a specific SHA-1 ref
gem "foreman", git: "git://github.com/pje/foreman.git", ref: "bf648a070c"
# ...or branch
gem "foreman", git: "git://github.com/pje/foreman.git", branch: "jruby"
# ...or tag
gem "foreman", git: "git://github.com/pje/foreman.git", tag: "v0.45.0"
(As @JHurrah mentioned in his comment.)