To flesh out your answer - starting from the right, the pedals and their cc codes are:
64 (Sustain, progressive, controller range 0-127), 66 (Sostenuto), 67 (Una Corda). These are principally intended for the Piano, though of course the 64/Sustain has been used to great effect for 'legato' playing for non percussive intruments in GPO1 thru 4 (in fact I struggle with other libraries that don't do this now).
64 and 66 work with the Aria Steiny, but, while the pedal lights up, I can't hear a difference with 67, so assume this pedal is there ready for when the mains Steinway migrates to Aria and perhaps the original GPO Piano didn't have the set of samples necessary to deliver this.
For version 2 I'd like all these to be implemented as continuous controllers (maybe the pedal light should get brighter) rather than on/off as they are now.
If you don't have three piano pedals/controllers, you can click on them with your mouse to light them up/engage them.