Recent data on comparative osmotic deswelling of poly(acrylic acid) gels and solutions shows that the difference between osmotic pressure of the gel and of the solution, P-gel(c, f) - P-solution(c, f), taken at equal polymer concentration(c) and fraction of the ionized acrylic acid monomers (f), changes its sign from negative to positive as the concentration c increases; this effect is enhanced with increasing f whereby it is shifted toward lower concentrations. In order to explain this effect, a model is suggested for the elastic pressure of the gel network which takes into account the effect of topological constraints combined with the effect of network ionization. According to the model, the sign-change of P-gel - P-solution originates from the topological constraints on conformations of the network; the ionization of the network chains enhances this effect and shifts it to the concentration range of the experiment.