We propose the GProbit model as an alternative to gravity models to estimate grouped-data flows. This is a model based on the random utility theory, which is consistent with the principle of population behavior. Instead of migrant counts, the dependent variable consists of a number of observed proportions. It allows explaining the propensity to migrate from any origin to a destination, which is an interesting relative concept not affected by the size effect. For this reason, it is expected to have better fit and less problems of non-normality, as illustrated by an application for migration flows of the Spanish regions.