Welcome to the forum. No, a midi keyboard is not totally necessary. I have a midi controller, but since I'm such a poor keyboard player, I only use it maybe 10% of the time. I normally just enter my notes into my notation program with the computer keyboard and that works quite well for me. I adjust the dynamics, articulations, slurs, hairpins in Finale and playback from there. If I need to tweak further, then I export the midi file from Finale into a sequencer and use those tools to refine my performance.
Welcome. I agree with Jeff 100% - you do not need a midi controller to get the most out of GPO (or any other VST). This holds especially true if you're a crummy keyboard player. Sibelius and Finale have become extremely adept at imparting good playback control to GPO, like Jeff mentions, and they create an excellent jump-off point for further tweaking in a sequencer.
As for samples of what is done with GPO sans keyboard, check out the listening room posts. Many, many users who post works use Sib or Finale's render exclusively and will mention it in the descriptions of their pieces.
I've written 60-odd pieces in Finale, and about 80% of the time I use one of the entry methods within Finale rather than using my MIDI keyboard. I would miss my MIDI keyboard if I didn't have it, no doubt, but mostly because it helps me enter things faster, not because it makes the finished product any better. If you don't have decent keyboard skills, you can absolutely get along without it.