The gem is nova_git_stats

Update the gem name
It is https://rubygems.org/gems/nova_git_stats

README.md

@@ -26,17 +26,17 @@ It browses the repository and outputs html page with statistics.
 
 ### Existing ruby/gem environment
 
- $ gem install git_stats
+ $ gem install nova_git_stats
 
 ### debian stretch (9.*)
 
  # apt-get install ruby ruby-nokogiri ruby-nokogiri-diff ruby-nokogumbo
- # gem install git_stats
+ # gem install nova_git_stats
 
 ### Ubuntu
 
  $ sudo apt-get install ruby ruby-dev gcc zlib1g-dev make
- $ sudo gem install git_stats
+ $ sudo gem install nova_git_stats
 
 ## Prerequisites