The GP(I) zone formation during quench is simulated in an industrial Aluminum alloy AA7449 75 mm thick plate by using a multi-class precipitation model. For this purpose, results of in situ SAXS experiments are reported. A methodology is presented that takes advantage of the collected data to derive i - a thermodynamic description for GP(I) zones from reversion heat treatments by using a solubility product and ii - the influence of excess vacancies on diffusion coefficients. This approach allows reproducing reasonably well the GP(I) zone formation measured during rapid cooling. Further, the simulated as-quenched surface yield strength compares well with experimental results reported in the literature.