Actually, Gemstone was created before Dragonrealms. Both offer the same world, slightly different races and classes, but in a 100% fantasy realm.

The difference basically consists of the system they implement. For example, Gemstone is more level-based while Dragonrealms is skill-based.

Personally, Dragonrealms was my favorite due to the skill-based learning and advancement (remember a time I spent hours swimming in ponds around the worlds in order to increase my swimming skills, a lot of fun if you're with a lot of people and roleplaying at the same time).

I find it amuzing that Massively posted something about Dragonrealms, another reason why I LOVE this site.