+1 for including the documentation patch, or 

2013/6/14 Bradley Lucier <lucier@math.purdue.edu>
 
I sent a patch to the Gambit mail list in 2009 that documents a bit more of this in _num.scm, but it was never applied:

https://mercure.iro.umontreal.ca/pipermail/gambit-list/2009-September/004020.html

Brad