I’ve had a GR-33 Guitar Synth for some years, which I use with my Godin Synth Access guitars. The GR-S has nothing like the features of the GR-33 – it essentially has 4 sound/affects: Crystal, which is a bell-like synth sound; Rich Modulation – an enhanced chorus effect; Slow Pad, a warm synth, with octave control; Brilliant Clean adds additional octaved strings (above and/or below) for a 12 or even 18 string effect. There is also a brilliant freeze function, which allows you to hold a pad on the synth, while continue to play on the guitar.
Although that does not compare in any way to the 300+ sounds in the GR-33, it does represent most of what I actually used the GR-33 for. More significantly, unlike the GR-33, the GR-S can run off battery power; which is the main reason for my purchase.
I love adding soft synth sounds to my guitar, but have never had it in a portable battery-powered form before, allowing me to use it in informal circles. The GR-S, together with my Blackstar Fly battery amp, means I can play with my Godins more in the future.