http://fs.fai.org/trac/wiki/ScoringFormulas

The most current documentation on Gap as implemented in FS.
GapGuide-2011-v1.pdf

The original GAP 2002 explanation from Angelo.
GAP02_en.pdf

Another explanation on GAP written by Nigel Leigh (NZ).
Thegapguide.pdf It's a good guide that explains the features of GAP quite
well for pilots.