PPGTT is short for the Per-Process Graphics Translation Tables. [1]
It is a feature supported by Intel's Ivy Bridge+, enabling GPU process isolation. This enhances the security by providing a per file descriptor address space / context.
Phoronix has been covering the process of PPGTT development [2]
[1] https://www.phoronix.com/scan.php?page=news_item&px=MTUzNzQ