Major, I'm a member of the GGC.
The rifle range does go out quite a way, but I can't verify the yardage. 300yd doesn't sound unreasonable, though. You can't shoot farther than that last berm as there are woods and houses to follow.
The pistol range is 25yd and has a shorter 55' range. All shooting positions are covered. IDPA takes up 5 "bays" near the end of the rifle range.
The quality of the people is the same as anywhere else, I guess. I've never run into a rambo or knowitall. Crowding can be a concern during "prime" hours on "prime" days. During the week or early in the morning, you pretty much have the place to yourself.
Truthfully, it's the best game in town. The next closest open range is in Anderson, but I don't deal with Gilmer and wouldn't recommend that you do. I hear that S'burg has a range, but I've never been there.