How does Gamma scalping really work or really mean? It seems there is no true profit scalped. If we look at the simplest scenario, Black-Scholes option price [$]V(t,S)[$] at time [$]t[$] and the underlying stock price at [$]S[$] with no interest, the infinitesimal change of the overall portfolio p&l under delta hedging, assuming we have the model, volatility, etc., correct, is

$$0=dV-\frac{\partial V}{\partial S}dS=\big(\Theta+\frac12\sigma^2S^2\Gamma\big)dt.$$

So the Gamma effect is cancelled by the Theta effect. Where does so called Gamma scalping profit come from? Is this just a folkloric myth stemming from misconception of how options really work or a gem in the rough?

Note: My condition implies that

$$ P\&L_{[0,T]} = \int_0^T \frac{1}{2} \Gamma(t,S_t,\sigma^2_{t,\text{impl.}})S_t^2( \sigma^2_{t,\text{real.}} - \sigma^2_{t,\text{impl.}})\,dt$$

coming from the misspecification of volatility is [$]0[$] since I am have assumed it away.