It would be very bizarre indeed if Mark were saying that the announcement of the arrival of the messiah was to be made exclusively to gentile individuals because in Acts there is every indication that Peter did not have any such commission and would have found that a scandalous suggestion at the time:
[Act 11:18 NIV] 18 When they heard this, they had no further objections and praised God, saying, "So then, even to Gentiles God has granted repentance that leads to life."
It seems to me that this passage provides the context for Mark's version of the commission:
[Act 2:38-40 KJV] 38 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. 39 For the promise is unto you [Jews], and to your [circumcised] children, and to all [the Jews] that are afar off, [even] as many as the Lord our God shall call. 40 And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation [the Jews of the last days of Judaism].
However, Paul WAS sent to the heathen, not exclusively but secondarily. That is, he would be sure to give the Jews their shot (though Paul, like all of the scriptures characterize the Jewish people as stubborn and intractable):
[Act 26:20 NIV] 20 First to those in Damascus, then to those in Jerusalem and in all Judea, and then to the Gentiles, I preached that they should repent and turn to God and demonstrate their repentance by their deeds.
[Act 13:46 KJV] 46 Then Paul and Barnabas waxed bold, and said, It was necessary that the word of God should first have been spoken to you: but seeing ye put it from you, and judge yourselves unworthy of everlasting life, lo, we turn to the Gentiles.
So since Mark uses the "divine passive" (a passive verb with no identified subject) he is NOT commanding the disciples aka apostles that THEY are to go to the gentiles because Jesus himself was never sent to the gentiles (though he could be convinced that it would not hurt his mission if a heathen received of the surplus benefits assigned to the Jews):
[Mat 15:22-24 CSB] 22 Just then a Canaanite woman from that region came and kept crying out, "Have mercy on me, Lord, Son of David! My daughter is severely tormented by a demon." 23 Jesus did not say a word to her. His disciples approached him and urged him, "Send her away because she's crying out after us." 24 He replied, "I was sent only to the lost sheep of the house of Israel."
So, we need to understand Mark's version of the commission to have been understood by the Twelve as either prescient of Paul's call or simply referring to the diaspora, to which Jesus had been sent and to whom he had sent the disciples, exclusively.