I can't speak right now about the difference. But when YSM Imports basically owned GA, they had the regs made in China, or Taiwan, or wherever they were made overseas. The weren't crap compared to Ninja, but they weren't made in the USA. You can look at threads from a few years back, and find that people were having issues with the Myth regs, but once they came out with the G2, which offered a low side burst disk, and made the original Myth regs with a low side burst disk, the only problems that you heard about with the Myth regs were the ping.
So are what really is the difference in quality? Ninja is made in the USA, GA was made overseas. They both offered a good reg that did what it was supposed to, was easy to change the output pressure, and user serviceable and rebuildable. I can't say what the GA customer service was like when GA was moved from the Chicagoland area to CA, but Ninja has always had incredible customer service.