not sure if this could help? I think this is a list of installed gems that use rack
paul@spyro:~/gems/gems$ grep -R \'rack\' * | grep -v ^rack
addressable-2.6.0/Gemfile: gem 'rack', '< 2', :require => false
addressable-2.6.0/Gemfile: gem 'rack', :require => false
addressable-2.7.0/Gemfile: gem 'rack', '< 2', :require => false
addressable-2.7.0/Gemfile: gem 'rack', :require => false
bundler-2.1.4/man/bundle-open.ronn: bundle open 'rack'
bundler-2.1.4/man/bundle-open.ronn:Will open the source directory for the 'rack' gem in your bundle.
bundler-2.1.4/man/bundle-inject.ronn: bundle inject 'rack' '> 0'
bundler-2.1.4/man/bundle-inject.ronn:This will inject the 'rack' gem with a version greater than 0 in your
bundler-2.1.4/man/bundle-open.1.txt: bundle open 'rack'
bundler-2.1.4/man/bundle-open.1.txt: Will open the source directory for the 'rack' gem in your bundle.
bundler-2.1.4/man/bundle-inject.1.txt: bundle inject 'rack' '> 0'
bundler-2.1.4/man/bundle-inject.1.txt: This will inject the 'rack' gem with a version greater than 0 in your
bundler-2.2.2/lib/bundler/man/bundle-open.1.ronn: bundle open 'rack'
bundler-2.2.2/lib/bundler/man/bundle-open.1.ronn:Will open the source directory for the 'rack' gem in your bundle.
bundler-2.2.2/lib/bundler/man/bundle-inject.1.ronn: bundle inject 'rack' '> 0'
bundler-2.2.2/lib/bundler/man/bundle-inject.1.ronn:This will inject the 'rack' gem with a version greater than 0 in your
rubygems-update-3.2.2/bundler/lib/bundler/man/bundle-open.1.ronn: bundle open 'rack'
rubygems-update-3.2.2/bundler/lib/bundler/man/bundle-open.1.ronn:Will open the source directory for the 'rack' gem in your bundle.
rubygems-update-3.2.2/bundler/lib/bundler/man/bundle-inject.1.ronn: bundle inject 'rack' '> 0'
rubygems-update-3.2.2/bundler/lib/bundler/man/bundle-inject.1.ronn:This will inject the 'rack' gem with a version greater than 0 in your