Well it does its job.
Right now I want to sound my line6 helix. I already have an active cabinet for my purposes so I consider gpa as a backup and for playing on not-mine gear.
This little guy powered my old ashdown 115 180W combo, and helix sounded better than thru loop return, obviously. It might be that ashdown's amp is broken, even better gpa did its work. :)
It also powered my other band's anonymous 115 & 410-s, normally powered by some taurus mini amp, and I could match sound levels with gpa. During rehearsal it was loud enough, against a loud drummer and 2x valve amp stacks. I believe it would work also in proper live situations.
This poweramp is indeed very small. I've fitted it into my rack's shelf, tied with zips. After 3h rehearsal, well, nothing burned, gpa was only mildly warm, room was not hotter than before.