This is just a guess...
If the pattern says that a certain # of stitches should measure X inches over k2 p2 ribbing, I would assume they mean UNstretched. The tank I am knitting now has a gauge measurement over St St, so when I checked my gauge I did it in St St instead of ribbing... and the ribbing is naturally coming out "tighter" than my swatch; I don't have enough to unstretch it yet (i hate ribbing it's too slow) but I

hope it comes out right....
Gardenia, is your project ribbing all over?
Vicki, does the pattern suggest a gauge in any other stitch besides ribbing? I don't know how to "slightly stretch" something... that does sound ambiguous.