As you've said, and just to be completely clear, in vacuum (neglecting, in other words, effects in macroscopic media like polarization), Gauss' law is the full, time-dependent expression of what you're calling the "first Maxwell equation."
The "derivation" of the Maxwell equations were originally formulated as differential (local) versions of the well known empirically observed laws of Ampere, Faraday, and Gauss. This is discussed some in Jackson's book ("Classical Electrodynamics"). Also see Griffith's book ("Intro to Electrodynamics").
The Maxwell equations aren't really derived from more fundamental considerations. Their integral form (the "laws" cited above) were deduced from observation, compared with phenomena not originally used in the determination of the empirical "laws," and found, in some regimes, to work.
In the regime of atomic physics, Planck found that the assumed continuous radiation of an accelerating charge predicted a black-body spectrum at large frequency in contradiction with that observed. And this led to a modification of the classical electrodynamics and the advent of the quantum theory.
The form of the Maxwell equations is, however, tightly constrained by invariance under Lorentz transformations. Jackson discusses this in Chapter 11.