You should install xcode-select packages first, then try installing nokogiri again. Try these commands,
xcode-select --install
then try
gem install nokogiri
with whatever Nokogiri version you want.
Nokogiri depends on multiple libraries like libxslt, libxml and zlib. Dev versions (including source) of these should be installed before installing Nokogiri in any Linux distribution. For OS X, the above command should work I guess.
The actual solution is in the comments below.