The GRG is a basswood body, 24 frets, with a trem. (Honestly the trem is probably completely useless though on a low end guitar like this). More suited for metal / hard-rock.

The GSZ is a mahogany body, 22 frets, fixed bridge. Mahogany produces a bit brighter of a sound with more sustain. Suits more music types from rock to blues.

There are reviews here:

Might want to consider some other amps. At lower end price range may want to look into the Roland Microcube, or a Cube 20x.