Consider a function $ f(x) $ on $F^*_p$ and $\Gamma \subseteq F^*_p$ is a subgroup.
The function is $ \Gamma$-invariant, meaning $ f(\gamma x) = f(x) $ for all $\gamma \in\Gamma$.
I'm reading the paper that says that due to the $ \Gamma$-invariance of the function, its 1-norm is
$$ \lVert f \rVert _1 = \sum_x f(x) = \vert \Gamma \vert \cdot \sum_{\psi \in F^*_p / \Gamma} f(\psi) $$
I don't understand how that equation came to be, considering that the $ F^*_p / \Gamma $ part doesn't seem to mean "quotient group". What meaning can $ F^*_p / \Gamma $ have in this context?