The only reason to add gliders to this game (accept for scenario purposes, but they were not generally -- if ever -- risked where an air2air threat was remotely likely) is if they can perform a task that the C-47 can't. So, what unique uses could they be used for? In addition to carrying troops, they could carry cargo, jeeps, artillery pieces, and (occasionally) light armored vehicles. They also had the attribute of costing a fraction of what a C-47 did, and could be piloted by minimally trained soldiers (a dozen hours of flight time versus hundreds for a powered a/c pilot). Remember also that C-47's were worth their weight in gold, according to Eisenhower and Kenny, who both felt the Skytrain was the most important air asset to their repective theaters. So, where does that leave us? Here's what I suggest...
1) Perk the C-47, such that it becomes the high-value asset it really was. Not real high, maybe 10 or so points, so that they're still relatively affordable...just not so expendable. They would still be capable of all the current payloads.
2) Add gliders (zero perk cost). With a glide ratio of 14:1, as suggested by Forrest's data, taking one up to 10K before releasing it would give it a maximum gliding range of 26.5 miles; that's better than one sector, and enough to keep the C-47 out of the most hazardus airspace around the target. It would be capable of all the same tasks as a C-47, but could also carry:
3a) 2 Jeeps or 1 halftrack (with any currently available options);
3b) 1 Jeep with a towed howetzer;
3c) 1 M-8, with glide ratio cut in half (i.e. 7:1).
With these additions, the glider has a unique purpose, with built in incentive to choose it over a Goon. Plus, it would open up interesting capture dynamics, where spawn camping the GV spawn points is no longer a sure-fire way to prevent a ground assault.