This be a failing on my part, I originally imagined Gravity as manipulating gravitational fields, though changing the mass of an object is a part of gravity, since that's what generates those fields. I'll say at first it'd be overwhelmingly hard to focus on changing something's mass, and only able to focus on a broad scale. (In essence you'd increase the mass of your entire body) Later down the road as your power grows and develops something like that might be possible