@rubygems is tweeting the description of the gemspec for each release.

So you can add hashtags to your description and I also recommend to add a link to a Diff of the CHANGELOG.md at the end if present.

voluntary.gemspec

Gem::Specification.new do |s|
  ...
  s.summary     = 'Crowdsourcing management system for Rails'
  s.description = '#Crowdsourcing management system for #Ruby on #Rails changes: http://bit.ly/voluntary-0-2-3'
  ...

The tweet looks like this

P.S.: How to get a "rich diff" of a CHANGELOG.md

Visit the GitHub show page of the commit and then copy the address of the link "Display the rich diff" shown in the image:

You now need a short customized URL e.g. from bitly (you need to be signed in).
Then you can create a new short URL by pasting the address in the field at the top with the placeholder "Paste a long URL here to shorten...", press enter, input the gem name plus version like this "#{name}-#{major}-#{minor}-#{patch}" in the keyword-field with the placeholder "customize" located in the right sidebar and press "Save".