This is all speculation, as I do not know about the history of GA.
GA seems to be a company which serves American sartorial Anglophiles. My guess is that GA is like Thomas Pink in that it is not an old British company but rather a newer one with good marketing.
That said, I have seen GA products (shirts and ties in particular), and they are leagues ahead of Pink in terms of quality. I would put GA products above Lewin (which is above Tyrwhitt and Pink) in quality, but perhaps just below H&K or T&A.
GA's offerings, especially their shirts, are a bit too loud for my taste (on average), but some are simple and discreet.
