"think carefully about" or "be cautious about." Gamaliel was warning them not to do something that they would later regret.
# Theudas rose up
Possible meanings are 1) "Theudas rebelled" or 2) "Theudas appeared."
# claiming to be somebody
"claiming to be somebody important"
# He was killed
This can be stated in active form. AT: "People killed him" (See: [[rc://en/ta/man/translate/figs-activepassive]])
# all who had been obeying him were scattered
This can be stated in active form. AT: "all the people scattered who had been obeying him" or "all who had been obeying him went in different directions" (See: [[rc://en/ta/man/translate/figs-activepassive]])
# came to nothing
This means that they did not do what they had planned to do.
This means that he persuaded some people to rebel with him against the Roman government. AT: "caused many people to follow him" or "caused many people to join him in rebellion" (See: [[rc://en/ta/man/translate/figs-idiom]])