Hi!
Indeed, this is very nifty. I also note that this uses some special features of our SPARQL endpoint that I did not know about (the "gas service"). It seems that this is a proprietary extension of BlazeGraph, which comes in very handy here.
Yes, it's described here: https://wiki.blazegraph.com/wiki/index.php/RDF_GAS_API and it's a service implementing basic graph algorithms such as BFS, shortest path, PageRank, etc. I personally didn't use it too much but it may be very useful for tasks which are naturally expressed as graph traversals.