The first Mac (a 128) was released to the broader public in January 1984. Gates undoubtedly had one much earlier, as Microsoft was writing software for Apple at the time. My recollection is that Jobs and Gates were collaborators on a number of things, so it is almost certain that Gates had one in 1983.
Whatever the timing, the computer behind Gates and to the right is a Mac (probably a 128 or 512).
I was refering to Prime Choice's recollection that his college had Macs in 1983, not Bill Gates's possession of one then. Also, it's already been established that the pictures in question came from 1985, not 1983 (note the touting of Windows 1.0, which came out in 1985).
That having been said, that definitely is a Mac (and it would have to be either the 128 or 512; they and the very-short-lived Mac XL, which looks a LOT different than the other 2 models, were the only models available in 1985).