A peek at
Jamo's website reveals that GTS is an economy unmodified (no latex additives) thinset. It's OK at best, as economy means cheap, and cheap meas that there is a minimum of portland cement in the product.
VersaBond is a modified thinset. It contains both latex additives that improve the properties of the mortar, and a reasonable amount of (expensive) portland cement.
And, at least in white, VersaBond is JB's favorite.