In the 1930s Arthur Brooke of 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 food being digested. Grocers abbreviated it to PG.
After the Second World War, labelling 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