You can add a file like this into your package:
Gemfile.local.your_package
gem 'rqrcode'
If the package (zpm) contains files like Gemfile* then it will show up additional commands in the UI of Admin → Package which will be required after package installation.
Related core code:
zammad/Gemfile at 31c03b93eed085ce184deb73a7288c904e5d2ffb · zammad/zammad · GitHub
zammad/app/models/package.rb at 31c03b93eed085ce184deb73a7288c904e5d2ffb · zammad/zammad · GitHub
