Exactly we can't replace gambling with trading even though their both have similar risk level but then the two are different in operational manner and reason.
While forex trading is done with the sole aim of making profits, gambling on the other hand have dual purposes.
Gambling is done for the entertainment of it and as an enjoyable activity and at the same time gambling can be done for the profit making that is why some players take it as a passive income method but with high risk though.
Most people perceive gambling more for pleasure and a good time spending. Gambling does not require special knowledge. Forex trading requires special knowledge and if there is no knowledge, then you will constantly lose money.