Chelsea had previously resisted his attempts to leave the club.
"William requested extra time off following the World Cup but was informed he should still report to Los Angeles on the date previously agreed with him," a club statement read on Tuesday.
FOOTBALL 2006 World Cup France Gallas
Image credit: Imago
Chelsea consider selling Pulisic amid interest from Premier League rivals – Paper Round
"He did not report and further action will now be considered by the club as a consequence.
"Pre-season training and the tour is about building team spirit among our squad and extra leave is only granted for valid football-related reasons."
Gallas, 28, has often made public his displeasure at being asked to play at full-back when Chelsea have suffered injury problems to Asier del Horno and Wayne Bridge and admitted to French sports daily L'Equipe that he would like to move to Milan.
ANGLING FOR MOVE
Whether this precise move is likely after Milan's punishment for Serie A match-fixing is open to question, but Gallas' actions may now force boss Jose Mourinho's hand.
Arsenal had previously expressed interest in the versatile defender, and with Sol Campbell no longer a Gunner and Philippe Senderos out for the start of the season there is definitely a vacancy in Gallas' preferred central role.
Gallas joined Chelsea from Marseille for around 9 million euros in 2001 and has one year left on his contract. He played in France's World Cup final defeat by Italy last month.
Mbappe hits brace and sets records as France qualify for last 16
Women's Champions League
Chelsea beat Real Madrid to stay perfect in Women's Champions League
Share this article