Back when gel spun backing first hit the market, I loaded several reels with it (this must be 13 or 14 years ago). To attach it to my fly lines, I used a short 6"-8" section of braided mono (30# or 50#) that was placed over the fly line with 3" or so of gell spun layed along side the fly line. The braided mono had about 1" of only fly line and about 2" of only gel spun on either end. I used tying thread to lock the ends to the line and the gel spun and also used tying thread to cover about 1/2" of the middle area where the gel spun and fly line overlapped. I coated the thread wraps only with very thin aquaseal. I never had one come loose, even on 35lb kings and 20lb chums.
On two lines, I used a uni-knot (double the gel spun to make it secure and insure it won't slip) on both the gel spun and 30lb dacron to tie them together. I made the uni-knot on the gel spun with the 30lb dacron in the center of the uni-knot loop, and pulled the knot tight. I then made another uni-knot on the 30lb dacron with the gel spun in the middle, and pulled it tight. The final step was cutting off the tag ends and then simply pulling the lines so that both uni-knots were butted up against one another. I never had this know slip of break either. I attached the short dacron portion to my fly line with an Albright knot. This was far easier and took less time than the braided mono method and was just as strong.