For simplicity, let us consider $U(1)$ gauge theory without matter fields.
At classical level, the gauge field $A^\mu$ has the gauge transformation law \begin{equation} A^\mu \to A^\mu +\partial^\mu \chi \end{equation} for any smooth function $\chi$. And the field strength tensor $F^{\mu \nu}=\partial^\mu A^\nu-\partial^\nu A^\mu$ is invariant under the transformation.
After a quantization like the Gupta-Bleuler one, $A^\mu$ become operator-valued distributions on some "physical" Hilbert space satisfying Lorenz condition $\partial_\mu A^\mu=0$ in that Hilbert space.
Now, my question is: How is the gauge transformation defined for the quantized fields? What does it mean by gauge-invariant "operators"?
My guess is that the gauge transformation is simply $A^\mu \to A^\mu +\partial^\mu \chi$ as operators on the Hilbert space and $F^{\mu \nu}$ is a gauge-invariant "operator".
Could anyone please clarify?