I have been looking for a way to see what gem uses a gem, so I can see examples of integration in other projects. The rubygems API guide don’t tell anything about such reverse dependency query. But it is actually there, it got merged some time ago, and optimized, but it is not documented yet (it runs really fast, kudos Rubygems guys).

ruby -ropen-uri -rpp -ryaml \
     -e 'pp YAML.load(open("https://rubygems.org/api/v1/gems/rails_best_practices/reverse_dependencies.yaml"))'

["metric_fu",
 "flyerhzm-metric_fu",
 "edouard-metric_fu",
 "devver-metric_fu",
 "goldstar-metric_fu",
 "socializer",
 "trollface",
 "guard-rails_best_practices",
 "rferraz-metric_fu",
 "git-hooks-helper",
 "odor",
 "rake_check",
 "koality",
 "danmayer-metric_fu",
 "bf4-metric_fu",
 "metrics_satellite",
 "code_hunter",
 "kinit",
 "rails-audit",
 "pronto-rails_best_practices",
 "free_disk_space",
 "warder",
 "ruby_osx_app",
 "sanelint"]

Out of curiosity I counted some wellknown gems usages by adding a .count at the end:

rake:    23766
rails:   6283
thor:    2786
pry:     2870
sinatra: 1964
devise:  422