God would not contradict himself. If he was really the angel Gabriel he would have said, “Don’t be afraid, Mohammed. I am Gabriel, and I am sending you a message.” But he didn’t. So if you analyze the situation, that spirit was not Gabriel by any means.

Compare how the angel Gabriel of the New Testament came to people of the Bible,
The Bible: Luke 1:11-13
11 Then an angel of the Lord appeared to him, standing at the right side of the altar of incense.
12 When Zechariah saw him, he was startled and was gripped with fear.
13 But the angel said to him: “Do not be afraid, Zechariah; your prayer has been heard. Your wife Elizabeth will bear you a son, and you are to give him the name John.
The Bible: Luke 1: 26-31
26 In the sixth month, God sent the angel Gabriel to Nazareth, a town in Galilee,
27 to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin’s name was Mary.
28 The angel went to her and said, “Greetings, you who are highly favored! The Lord is with you.”
29 Mary was greatly troubled at his words and wondered what kind of greeting this might be.
30 But the angel said to her, “Do not be afraid, Mary, you have found favor with God.
31 You will be with child and give birth to a son, and you are to give him the name Jesus. by Dr. Labib

The phrase “Do not be afraid,” is a signature of God used by all angels from God.
This Gabriel was not the same Gabriel of the Bible.