Gaia Science Alerts microlensing events with mass estimates verifiable in Gaia Data Release 4

Gaia Science Alerts provide an all-sky stream of photometric transients, including events consistent with gravitational microlensing. For such events, forthcoming epoch astrometry from the Gaia mission offers the possibility of testing photometric lens mass predictions. While microlensing parallax inferred from the photometric light curve can constrain the event geometry, a direct lens mass measurement additionally requires the angular Einstein radius, which may be accessible through astrometric microlensing. We aim to construct a pre-Data-Release-4 (DR4) sample of Gaia microlensing events for which the available photometry provides a measurable microlensing-parallax signal. This allows us to derive statistical predictions for the lens mass and distance. We forward-modelled these events to predict the detectability of the astrometric microlensing signal in Gaia DR4. We identify 56 Gaia Science Alerts microlensing events with 71 point-source point-lens solutions with annual parallaxes that satisfy our fit-quality, posterior-boundedness, and parallax-significance criteria. Lens masses and distances were inferred using two prior-based frameworks, pyLIMASS and the Besançon Galaxy Model, which combine the measured microlensing parameters and Gaia source information with stellar-population and Galactic models. These inferred parameters were used to simulate Gaia DR4-like along-scan astrometry and estimate the detectability of the astrometric microlensing signal. The inferred lens masses and distances remain strongly prior-dependent without a direct angular Einstein radius measurement. Among the 284 event--solution--prior branches, 47 are classified as high-confidence astrometric detections using Gaia DR4-like simulations. These predictions can be tested once epoch astrometry is released in Gaia DR4.