Basic answer is the one ExCon90 gave. The GG-1 was designed and built for use under AC overhead. It would have taken major modifications to allow them to operate off third rail (or from a DC overhead), and none, so far as I know, were ever modified in that way.
Railroads other than the PRR who operated GG-1 were Penn Central, Conrail and Amtrak, who inherited them by merger or under the complicated arrangements of setting up Amtrak. They used them on the ex-PRR lines and ex-New Haven Railroad lines with AC overhead power compatible with their design.
Possible confusion: The New Haven's EP-3 and EP-4 electric locomotives (built by GE, with running gear similar to a GG-1's but very different carbodies) WERE capable of running either under AC overhead wire (north or New York and on the Hell Gate Bridge line into Penn Station) or on third-rail DC power: so there were locomotives similar to a GG-1 that did have this dual ability.