Gabriel is also the author of the popular Pipes Haskell library, and nice person in general. Both of these make me heartily recommend his other blog posts about various Haskell topics, which often portray advanced features remarkably well.
Agreed. I sent him an email asking for help on proving some category theory laws for some datatypes, and he wrote back a several page email answering everything in detail.
Examples:
Scrap your type classes: http://www.haskellforall.com/2012/05/scrap-your-type-classes...
Hello Core: http://www.haskellforall.com/2012/10/hello-core.html
Coding a simple concurrent scheduler yourself: http://www.haskellforall.com/2013/06/from-zero-to-cooperativ...
Tutorial as part of library doc (!): http://hackage.haskell.org/package/pipes-4.1.1/docs/Pipes-Tu...