I'm using this gem on a couple GeoBlacklight 2+ / Blacklight 7+ installs. The gem's generator adds a saved_searches_controller.rb file referencing Blacklight::SavedSearches:
https://github.com/projectblacklight/blacklight_advanced_search/blob/master/lib/generators/blacklight_advanced_search/templates/saved_searches_controller.rb
I believe Blacklight::SavedSearches was removed in BL 7... so for this gem to run correctly, you have to delete the saved_searches_controller.rb file the generator creates like so:
psu-libraries/psulib_blacklight#237
Hope this helps someone!