The Gin Pole length isn't really critical - you are just moving the forces at more of an angle to allow the winch/block and tackle set up to work better. Diameter and quality of tubing is more critical. You can buy 6061, T-6 aluminum pipe ("tube" is WAY more expensive) in various outside diameters from steel supply outlets. I think mine for my boat (not a Getaway) is about 6 feet. Look at diagrams or pictures, figuring an average person is 5'8", in the US will give you a good idea about where you should be at. For an eye bolt, get a welded eye bolt our of stainless. 5/16 should do it.
Looks to me like the pole is between 5 and 6 feet:
https://media.hobie.com/item_attachments/3156z105.pdfHobie's directions are pretty darned good. If you want some more general ideas:
https://www.thebeachcats.com/index.php? ... opic=18048But the biggest thing to watch is to not get the rigging in such a bind that you snap something. That's bad. I love my setup, which allows me to solo-step my H21SE mast @33 feet, without too much fuss. Easier with 2 people, just moving the massive mast around, but I've done it as many times by myself as with 2.