Is it possible to do a case-insensitive `gsub`?

You can pass Regexp instead of String, with i option that indicates that this regexp is case insensitive:

def add_more_ruby(string)
  string.gsub(/bad/i, 'good')
end

note that it will substitute not only 'bad' and 'BAD', but also, for example, 'bAd'. If you want to substitute only all-uppercase or all-lowercase, you can do:

string.gsub(/bad|BAD/, 'good')