Gateway specs still depend on if you're serving vision or perspective though since perspective logic all runs on the gateway.
I tend to follow @pturmel guidelines for perspective here Microsoft Virtual Machine Suggestion - #5 by pturmel
Re vision I have found similar as @lrose . With best practices you can get away with the relatively weak thin clients.
It's really hard to say in a complete vacuum though. 16 gigs is probably overkill for 1 HMI style project and probably no where near enough for perspective projects with 100+ users.