Im trying to understand why everyone uses the Densglass Gold product. There are a million fiberglass gypsum boards, but this is the only board I see anywhere.
Does it perform better? Cheaper? What is the story here?
Sometimes cheaper is not better. Maybe its trust! or perhaps you should read the warranty. I have had success with other GP Products, Denshield, DensArmor, perhaps it runs in the family.
I think this is explained by the first-to-market advantage: If I’m not mistaken, GP invented the category, and held a patent until a number of years ago, giving them market exclusivity. Since the patent expired and other companies began offering similar products, GP has retained some of its advantage thanks to habit and good marketing.
Others are gaining market share, though – I see purple, green, and other boards in addition to the familiar gold.
Jeffrey is correct, additionally over the years Georgia Pacific has had some excellent Reps.
Jeffrey, color doesn’t mean much anymore, look at Huber’s Zip product, it has a green facing, from afar it looks like green board, hard to tell its actually OSB.
Color can be essential to a brand identity. In intellectual property law, it is called “trade dress.” You know instantly who makes pink insulation, and who makes earthmoving equipment painted a particular shade of yellow.
Technically (and fully understanding hyperbole), there isn’t a million versions of fiberglass faced gypsum board. Realistically, there are probably less than 6 manufacturers. How many are distributed in any geographic area? Probably 3.
Around my area, it’s typically gold or purple…sometimes both on the same job! 
USG started selling a fiberglass-faced exterior gypsum product soon after G-P invented DensGlass Gold. G-P sued USG for patent infringement. USG had to wait 20 or so years for G-P’s patent to run out before resuming producing their competitive product.