This study investigates the early stages of precipitation in Al–Zn–Mg alloys using an atomic cluster expansion interatomic potential, with a focus on the atomic structure and stability of Guinier-Preston I (GP-I) zones. Formation energies of solute clusters and precipitate variants are evaluated by static calculations, and precipitation is further explored via large-scale atomistic simulations of supersaturated solid solutions. Simulation annealing leads to the formation of spherical GP-I zones composed of high-symmetry blocks, with Zn atoms frequently shifting toward interstitial positions. The formed GP-I zones consist of a Zn-rich core surrounded by a Mg-enriched interfacial region, resulting in average Zn/Mg ratio of approximately 1.5 for the precipitates. The simulation results indicate that increasing temperature promotes the dissolution (reversion) of pre-existing GP-I zones instead of their gradual transformation into more stable precipitates. Upon dissolution at elevated temperature and in the presence of vacancies, stable vacancy/divacancy-solute complexes are formed, highlighting the key role of vacancy-solute interactions in early clustering and aging response.