Why it smells like gas inside car when heat is on?

There has to be a gas leak under the hood. The fresh air intakeor even when re-circuklating will come from the back top of the motr mostly.
The problem with finding small gas leaks is that they evaporate faster than you can find them. You may have to wait for it to get worse.
A common leak is the injector seals. Check around the injectors for leaks.