"PA GG-1" refers to the Pennsylvania Railroad GG1 electric locomotive .
"Grit Gauze" indicates a mesh with a relatively thick thread diameter and a square weave, making it durable enough to handle heavy loads and "gritty" particles. Typical Specifications (from PDF data) 1 PA GG pdf
While "PA-GG" is a milling standard, the acronym can occasionally appear in other contexts: "PA GG-1" refers to the Pennsylvania Railroad GG1