It's the gas gun that Browning replaced the A500R with, that itself was replaced with the Gold Series a year later.
Not much value (certainly not $600) due to not much demand (mostly), and repair parts are unobtanium since it was disc over 20 years ago, after only that one year's production.
The A500G died on the rep created by the A500R, as evidently, most shooters couldn't distinguish any difference.
If you decide to buy it/one, and someday need repair, the only gunsmith left alive that knows them inside/out IIRC is Art's Gun Shop (google).
Other than that, I'd keep one maintained like any other steel-receivered gasgun.