I'm actually working on a one point hitch for the front of my b6100 that will accept Gravely 2-wheeler UNPOWERED attachments.
My stockpile of attachments is getting larger by 3 this saturday.
The easiest way I can see to power a Gravely attachment off a rear tractor pto is probably obtaining a Gravely rider to 2-wheeler adapter hitch and modifying it for a 3 point hitch.