Skeeto/Chris Wellons [0] did a great write up on a basic Reference Counting Garbage Collector. I've done an initial translation [1][2] into a reusable library. Bob Nystrom also wrote a great tutorial on implementing a basic GC for a simple dynamic language [3].

[0] http://nullprogram.com/blog/2015/02/17/

[1] https://github.com/lpsantil/vgc/blob/master/original/t/t0000...

[2] https://github.com/lpsantil/vgc/blob/master/original/include...

[3] http://journal.stuffwithstuff.com/2013/12/08/babys-first-gar...