My impression is MIT is the most widely used license (e.g. Rails, Rack, RSpec) followed by BSD (e.g. puma) and GPL (e.g. unicorn). Also, please note Ruby itself is dual-licensed under 2-clause BSD or the Ruby license (https://www.ruby-lang.org/en/about/license.txt).
I think the right license depends on what's your goal. If you'd like your software to be used in the wild then permissive licenses like MIR, BSD, Apache, etc. are preferable.