This kinda almost makes it sound like VisiOn introduced Gates to the GUI which is not true. They were already using and developing for Macintosh machines at Microsoft and had hired Charles Simonyi from PARC with his expertise in GUI systems before Comdex 1982.
The story seems to be more that seeing VisiOn gave MS the nudge to get into action on the GUI due to a sense of competition/urgenbcy from VisiOn rather than learning about the GUI from VisiOn.
Stories about Xerox PARC aside, the Apple Lisa was released to the market at the end of January 1983, and I would be surprised if Microsoft wasn't big enough/rich enough at that point to acquire at least a couple of units from the very first batch to mess around with. Windows 1.0 RTM was November 1985.
The story seems to be more that seeing VisiOn gave MS the nudge to get into action on the GUI due to a sense of competition/urgenbcy from VisiOn rather than learning about the GUI from VisiOn.