The Welsh forward scored as a substitute in the 3-1 victory over Atletico Madrid, bringing up his 100th goal for the club.
And during the celebrations that followed, Bale "lifted his right arm near his head in a clear sign of provocation towards the fans, then giving a gesture which could be seen as obscene and disdainful by folding his arm and striking it with his other hand."
In a statement released by La Liga, Bale’s punishment will be decided by whether he incited the crowd or not.
If Bale is guilty of “inciting opposition fans” to cause a hostile reaction, the punishment could carry four to 12 games, if there was no hostile reaction but intent, then it could be as little as one to three games.
The gesture is referred to in Spain as ‘corte de mangas,’ with the moment not picked up in the referee’s match report.
A committee will now decide whether the gesture merits a punishment.
Bale also featured in Real’s Champions League victory over Ajax.
Los Blancos take a 2-1 lead back to the Bernabeu for the second leg of the last 16 tie after Marco Asensio’s late winner.