Bill, yeah, GTO15 is kind of like sparkplug wire. Have a very heavy insulation around the conductor. I really looks like coax, but there is not outer conductor (I am sure you looked it up and saw that). The GTO15 is part of the antenna. So the total length of the backstay, plus the GTO15, is the antenna. The highest current point is at that piece of GTO15/chainplate.
Get, just for fun, try cutting a radial for the primary frequency you operate on. Attach it to the lug and let it lie around the area of the tuner. If possible, stretch it out but if not, coiled will work also. Here is how I have my setup at home which mimics what I might do on my boat. I have a 36' length of wire as my antenna. Connected to a 4:1 UNUN balun. Connected to that is a current balun to keep the RF from folding back on my coax. Then a length of coax to an autotuner, then the rig. The other side of the 4:1 is a 25' length of wire dropped down and just lying around (kind of like a very very very simple KISS system). That's it. I get out OK enough to make contacts. Not going to win any contest, but good enough.