I wrote a Rust tool to process the downloaded hash list into a compact (29GB -> 1.5GB) database that can be efficiently queried: https://github.com/Freaky/gcstool

I made a start on a Ruby port too: https://github.com/Freaky/ruby-gcs - I have vague plans to finish it off and write a Rodauth (http://rodauth.jeremyevans.net/) plugin for it.