The leak was first reported around 9:15 a.m. on the 1900 block of East Madison Street in the city's Kensington section.
When fire crews arrive they confirmed that gas leak but couldn't find where it was coming from.
Twenty homes were evacuated while the fire department conducted their investigation.
The situation was brought under control at 10:12 a.m.
PGW believes that the leak was in the street and will dig it up and fix the problem.
No injuries have been reported.