In roman law, when a militar was accused of treason, what was the penalty?

It would depend on the level of the crime and the status of the person.

First of course there was a trial, but it was more show than law.

Sometimes they would be offered an honorable way out, like falling on ones sword or poison. sometimes the family would be there for a final goodbye.

other times it was public execution, garroting was a common method of execution.

Slavery was rarely an option for men of high status although his family may be subject to it on occasions.

Possessions and land holdings may be turned over to the senate, but it again would depend on the method of death, if it was honorable then perhaps the family kept it, if not then it could be stripped.

there was not really a normal way to handle these cases, it was really on a case by case basis.

His possessions were confiscated by the state. Happened a lot. One way to balance the imperial budget was to outlaw one of the ten richest men in the Empire and seize his possessions.

Often the members of his household would be sold into slavery unless some friend "bought" them. Otherwise his wife and children might be killed. In a few cases, the wife's political connections were enough to keep her safe. She'd be told to divorce the scoundrel, of course.

The man himself was usually executed. If he was a Roman citizen, he had a right to die by the sword. If he wasn't, the cross was always there. He wasn't sold into slavery since as long as he was alive, his friends could rally around him and turn treason into a coup.

He would have his Lands ,houses and wealth confiscated unless he took his own Life with his sword , this would create honour instead of shame and therefore protect the rights of his family and their personal wealth and family name.If he couldn't do this he might have asked a friend or family member to do it for him.If there was no action taken the senate would order his arrest and Execute him.Then the family would suffer the above consequences.

Weren't they hanged, or stoned to death?

He is probably executed and all his assets seized...

he becomes a slave and/or gets killed