Make sure you are using ruby 2.6 or 2.7
And I recommend upgrading to jekyll 3.9
As that it what github pages uses and any maintained gems will work with latest Jekyll.
Listen to the error message. It is your friend.
Can you see which line of your file is failing? What does your line 7 see look like? Can you post a link to your plugin file in your repo.
In the script on github, i see a “<” in line 6 not line 7. And that is standard class inheritance so I don’t see an issue.
See if you can ruby the script alone to see if it is a ruby issue and not a Jekyll issue.
ruby _plugins/jekyll_minimagick.rb
# or
bundle exec ruby _plugins/jekyll_minimagick.rb