The Lumia 950 launched on At&t network in November, last year. It's available either on a 2yr contract or with At&t's Next program.
Today At&t added the Lumia 950 to its GoPhone service (no contract) for $598. There isn't really an incentive on getting it through GoPhone as it is the same price off-contract.
It really doesn't make sense as At&t GoPhone is usually used to sell really low budget smartphones like the Lumia 640 which now has a new price of $30 or $25.49 for the certified refurbished version.
If you are interested on getting the Lumia 950 or the ultra affordable Lumia 640, visit AT&t GoPhone