E. De Felice considers that it is a Lombard surname formed from the place name Gerosa, a locality of the Bergamo area. E. Rossoni agrees in pointing to that toponymic origin, but adds an alternative hypothesis: the surname could also derive from nicknames based on the Lombard dialectal word geros, meaning "stony" or "with gravel". Rossoni places its presence in the area comprising the Como, Lecco, Bergamo and Milan areas.

R. Brondolo proposes a different etymology: the surname would come from gèra, a dialectal term meaning "gravel", and would allude originally to an inhabitant of a stony place or one with pebbles. This interpretation converges partially with the dialectal variant noted by Rossoni, although Brondolo presents it as the direct origin of the surname, without the mediation of a place name.