Mass is just one consideration. How fact you're wanting to move it, what acceleration is at play, friction of bearings and guides (as applicable), belt pitch, momentum, etc all enter into the calculation. GT2 is just a belt profile. Larger pitches and wider belts can handle larger loads than smaller pitches and narrower belts. The larger the gear the larger the number of teeth and therefor the larger the load rating, but at a sacrifice of resolution as each step travels a larger distance due to the larger diameter. Without knowing what exactly you're doing it's not an easy question to answer.
