Gentiles do not have to adhere to Jewish customs such as circumcision of the penis, Jewish holidays, animal sacrifices, water baptism, etc. Jesus came to save the Jews, but when they rejected Him, the door was opened for Gentiles.
Acts 2:38-39 (Jews) The group did not receive the Holy Ghost the moment they believed in Jesus. However, they were told they could expect the Holy Ghost at some point.
Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost.
For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call.
Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls.
Acts 8:12-17 (Samaritans)
But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women.
Then Simon himself believed also: and when he was baptized, he continued with Philip, and wondered, beholding the miracles and signs which were done.
Now when the apostles which were at Jerusalem heard that Samaria had received the word of God, they sent unto them Peter and John:
Who, when they were come down, prayed for them, that they might receive the Holy Ghost:
(For as yet he was fallen upon none of them: only they were baptized in the name of the Lord Jesus.)
Then laid they their hands on them, and they received the Holy Ghost.
Acts 10:43-48 (Gentiles)
43 To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins.
While Peter yet spake these words, the Holy Ghost fell on all them which heard the word.
And they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost.
For they heard them speak with tongues, and magnify God. Then answered Peter,
Can any man forbid water, that these should not be baptized, which have received the Holy Ghost as well as we?
And he commanded them to be baptized in the name of the Lord. Then prayed they him to tarry certain days.