I'll forwarn you about using those gelspun lines. I'm going more off Powerpro, but is pretty much the same thing. If you don't carefully watch that line as you're putting it back on, you could have MAJOR problems with your reel AND rod. Nothing worse then losing the fish of a lifetime while breaking the rod too.
I've been using powerpro for a couple years now. Mostly on my baitcasters and spinning rods. Have used it a little in flyfishing, but not much. The problem with this stuff is the diameter. First, you have to do a TON of cranking to wind that stuff back on. Unless you're deep lining the salt, you won't need 200+ yards of backing (just think having a fish being TWO football fields away, usually won't happen and if it does say goodbye anyways lol). But the stuff is prone to wrap around it self. If you're fighting the fish and it's putting pressure on the backing, it'll slowly dig it's way into the reel/line. Bascially making a ratsnest (which is usually reserved for baitcasters). You'll have a good run of the fish and the line will stop. Problem is, it's strong stuff and just won't "break" on you. If fish is fighting hard enough, *snap* goes the rod. I had a similar experience last year fishing for salmon from a jet sled using powerpro as backing (helps having a friend who's a prostaffer for them lol). Had similar problem, luckily it was only on a 30# king, would've hate to have a big brute on the end. Luckily, I was using a glass rod, so took the abuse a bit more while I tried to free the spool. Thank god for years of working a baitcaster, i was able to pick the spool out quick while stripping in line. But wasn't pretty, and those superlines will cut into your hand while stripping in a big fish (if you are were in the position I just described where you can't really reel).
I am back just using standard dacron backing in 30# test. No need for all the extra room. Plus, when I'm cranking line back on, I don't want to have to think that much about line placement. Which is critical for this stuff.