Gelatin gels, permanently cross-linked following two different routes, exhibit the same phenomenology in which their turbidity builds-up in concert with their elastic modulus. This is analyzed in terms of a minimal model for the chemo-osmotic coupling mechanism first proposed by Bastide and Candau (in: ``Physical Properties of Polymeric Gels'', J.-P. Cohen Addad Ed., J. Wiley & Sons, New York 1996). The analysis points out the important role of labile counterions, which usually stiffen (osmotically) the gel, thereby hindering the coupling. This might be the reason why this a priori generic mechanism was not observed earlier.