Petrol is the name of a company the was removed from the final version of Grand Theft Auto III. It is not known what the company was going to sell or manufacture. Billboards and signs bearing the Petrol logo could be seen in beta screenshots for GTA III.
It is likely that the company's name was changed to Gasoline in the final version of the game. Various Gasoline billboards can be found around the city in GTA III, and when looked at from afar, they bear the Petrol logo as a low-density texture.