Just as “Patriotism is the last refuge of a scoundrel”, “Bad gas” is the default diagnosis by a mechanic who is stumped. As was already said, while “bad gas” is a possibility, it is a remote possibility.
And, as was also stated already, an engine that “revs” freely while not accelerating the car is not an engine with a problem. This is indeed the symptom of engine power not being transmitted to the drive wheels of the car, and that points toward a transmission problem (if it has an automatic trans), or a clutch problem (if it has a manual trans).