In the 1930s Brooke Bond launched PG Tips in the UK tea market under the name of Pre-Gest-Tee. The name implied that the tea could be drunk prior to eating food, as a digestive aid. Grocers and salesmen abbreviated it to PG.
After the Second World War, labeling regulations ruled out describing tea as aiding digestion—a property previously attributed to tea—and by 1950/1 the PG name was officially adopted. The company added “Tips” referring to the fact that only the tips (the top two leaves and bud) of the tea plants are used in the blend.
Info gleaned from Wikipedia