The GT2 is a very versatile distortion unit. It can go jazz clean to all out death metal without a booster. Its tone is more on the modern side. Analog modelling is also much warmer than digital stuff.
If you throw a DS-1 before the GT2, you can get a thick lead tone. Plus, you can combine the DS-1 before all the GT2 tonal possiblities, which is a ridiculously huge range.