An acronym. The "GPP" is the DOS-ified spelling of "g++"; the '+' was not (officially) allowed in MS-DOS filenames. The DJ stands for the first name (not the initials, apparently [1]) of the project's originator, DJ Delorie.

Although: "Since C++ is integral to gcc, djgpp no longer stands for "DJ's G++" but probably stands for something like "DJ's GNU Programming Platform"." [2]

[1] https://www.delorie.com/users/dj/

[2] https://www.delorie.com/djgpp/history.html