"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.
# After this man
"After Theudas"
# in the days of the census
"during the time of the census"
# drew away some people after him
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]])