class RuboCop::Cop::Packaging::GemspecGit

def investigate(processed_source)

Processing of the AST happens here.

https://github.com/rubocop-hq/rubocop/blob/59543c8e2b66bff249de131fa9105f3eb11e9edb/lib/rubocop/cop/cop.rb#L13-L25
More about the `#investigate` method can be found here:
Extended from the Cop class. 
def investigate(processed_source)
  xstr(processed_source.ast).each do |node|
    add_offense(
      processed_source.ast,
      location: node.loc.expression,
      message: MSG
    )
  end
end