As a a starting point, "The Origins of PostScript" (https://gwern.net/doc/design/typography/2018-warnock.pdf) provides a few details on the language and Gaffney's involvement. Warnock's oral history for the Computer History Museum (https://www.computerhistory.org/collections/catalog/10273875...) also includes the story. Gaffney's patent provides the most detail but, unfortunately, it is written as a patent rather than a language description.

The DoD DTIC service has a couple of reports that cover the Harbor Pilot Simulation, but I haven't found any reports written by E&S. The Computer History Museum has some records from Evans and Sutherland, but I don't think any of them cover the language.