Daniel, thanks for the clarification; I can see how, for an infinite charge distribution, Gauss's Law applies but is not equivalent to Poisson's equation.
I find it surprising that the equations in integral form are more fundamental than the equations in differential form. Contrast this...
Followup question
Claude, thankyou for casting some light on the matter! You mentioned that Poisson's equation is not applicable to charge distributions of infinite extent. Are Maxwell's equations defined for such systems? Also, do you know of a book which discusses this?
Consider a uniform charge distribution occupying all of (flat) spacetime,
\rho(t,x,y,z) = \text{constant} \;\;\;\;\; ,\; (t,x,y,z) \in R^{1,3}
Because this charge distribution is translationally invariant, it seems reasonable to expect that the electric field arising from the charge...