It does very "simple" things and does not need an OS per se: if you look at the source code, it's basically a loop getting GPS pos and dumping them.

For comparison, here is a (incomplete/outdated) list that shows smallest footprint for a Linux system is here this: http://elinux.org/Best_of_Embedded_Linux#Candidates

In other words: way too small to run any sort of modern day OS....(and it would be too power-hungry anyway :-) )