Congratulations on the steal of a deal you found! EZ3 frames on eBay are several hundred dollars, last I looked. I posted about this frame a few weeks ago and did some research on how to use it. One helpful site is quilting-tidbits.com. I don't think she mentions the Grace frame specifically, but there is info about attaching a leader using velcro that might help you:
https://www.quilting-tidbits.com/han...ing-frame.html
Another useful resource is Jinny Beyer's book
Quiltmaking by Hand (Breckling Press, 2004). See pp. 177-179 for info and good photos on attaching a quilt to a leader and loading it to a frame. Her frame is a traditional 2-rail wooden frame, but the same principle applies.
You might get lucky and find someone at the Grace Co who could advise you about attaching a leader.
The leader should be made of sturdy fabric, such as ticking, canvas, or duck.
Good luck!