Build gem from .gemspec to store locally

I am getting the error uninitialized constant for the vestal_versions
gem when I build it from a gemspec file and install it.

I need to store gems locally at a client site. I am using bundle
package for most of my gems but the vestal_versions gem comes from a
git repository and I can not included in the gems stored in vendor/
cache. I am new to building a gem from a gemspec file. What do I need
to change so that the Rails application can see the locally built gem?