According to Italian media reports, Gabbia missed training and is aiming to return for the match against Lazio.

AC Milan have resumed training to prepare for the match against Lazio; Gabbia was absent and hopes to rejoin team training tomorrow.
AC Milan have resumed training. The Rossoneri returned to preparations after drawing away against Juventus of Turin in the previous league match, and have begun preparing to face Lazio, coached by Gennaro Gattuso, who are leading the table after winning all three of their opening matches.
Amorim has an almost fully available squad, with only Gabbia absent. The defender is still troubled by an ankle injury and therefore missed the match against Juventus.
However, Gabbia's condition is improving. The defender hopes to return to training with his teammates from tomorrow and aims to feature in the match against Lazio.