In addition to being able to layer instruments, GPO uses the Mod Wheel for volume (for ww, brass. & strings).
Real instruments constantly change their volume. You should 'ride' the Mod Wheel while playing to make them
sound more expressive.
Velocity on ww, brass, & strings controls the attack type (legato/staccato), NOT the volume.
Velocity on all the other instruments (piano, harp, organ, percussion, string pizz, etc.) DOES control volume.
For it's price category, GPO offers a lot.