Valerie are both pretty popular plum shades for Zoya. I can totally see why, because to me I can barely tell the difference. Valerie definitely has more and larger glitter than Gem. Valerie's formula was also a little thicker than Gem. If you look closely, you can see the surface reflection in Valerie is a bit rougher. Gem is much smoother with a finer particulate of sparkle. Gem also applied a bit smoother with easier control.
I would say that you can own one or the other, but you probably don't need to own both. If you like more sparkle, go with Valerie. If you want something a little more muted, pick up Gem. Here is 2 coats of Gem with 1 coat of nubar Diamont.