Some companies will only use gems with a certain license.
The canonical and easy way to check is via the gemspec
via e.g.

spec.license = 'MIT'
# or
spec.licenses = ['MIT', 'GPL-2']

There is even a License Finder to help companies ensure all gems they use
meet their licensing needs. This tool depends on license information being available in the gemspec.
Including a license in your gemspec is a good practice, in any case.

If you need help choosing a license, github has created a license picker tool

I've written a blog post, as well: Make the world a better place; put a license in your gemspec

This is how I create issues:

1. From the stats I've collected, find all gems without a license specified in the gemspec
2. Map the result to a list of github 'username/reponame' guessed from urls specified in the gemspec
3. Remove from the list any gems with a username on the blacklist
4. Remove from the list any gems that I've already processed (whether I created an issue or not)
5. Check all the open and closed issues for the word 'license' or 'licence'
6. If not found, create an issue 'License missing from gemspec'
7. Update my list of processed gems so the same repo is never processed twice

I'm doing this via a script as a public service. :) So far, it's going pretty well. Apparently, I was mentioned on Ruby5.

also see

• surfacing licenses on rubygems.org - which includes a reference to most of the issues I created before creating this issue and repo.
• Rubygems guide recommends including a license in your gemspec
• adding a license to bundler-generated gems, defaults to 'MIT'
  • Added LICENSE (MIT) to newgem rubygems/bundler#1571
  • Default new gem's gemspec to have MIT license. rubygems/bundler#2100
  • commit
• Rails plugin new defaults license to MIT