The D100 system is a hybrid partial-success multiple-success system that provides the ease of conflict resolution inherent in a multiple-success system combined with the convenience of not having to roll large numbers of dice.
The primary difference between TGS and, for example, d20 is the way actions are resolved. A single die roll determines not only whether or not you succeed, but also how well you succeed. For example with a single d100 roll determines both whether or not your attack hit and how much damage it does. The resulting mechanic is similar to the way a multiple success system works, with opposed checks going to whoever gets the most successes. However, you don't need a large handful of dice; 2d10 is all you ever need.