I installed jekyll-minify-html and it works great as expected, but only on Pages. Posts are not minified, strangely.
I browsed the plugin source, it looks to me like it ought to work for Posts, but I'm not a Ruby guy, so I can't really debug it.
I don't know anything about that plugin, but have been using http://jch.penibelst.de/ for quite a while now and highly recommend it. It's not a gem, it's a layout that you add directly to your site.