I use both gel spun braids and dacron for backing. Most of my bluewater gear is rigged with gel spun because the thinner diameter and slicker line produces less line drag than dacron backing so I am less likely to break of fish sole due to the pressure of the line being dragged through the water. If using gelspun I would use at least 50lb. You are less likely to run into problems with the thicker braid and you still will get more backing on the reel than if using 20lb dacron. Make sure you wind it on fairly tight and criss cross back and forth on the spool every few wraps. If you do this you wont have any problems. If its too loose it can dig in. If you can spool it with a machine that is ideal. I usually spool it on to one of my conventional saltwater reels first then reel it onto the fly reel from the conventional reel with a very light drag setting. I have never had a problem.