There is something similar in Luke, chapter 8:39, while there is nothing like this in Matthew. Mark said that the former demoniac went away (καὶ ἀπῆλθεν). He then began to preach or proclaim (καὶ ἤρξατο κηρύσσειν) in the Decapolis area (ἐν τῇ Δεκαπόλει) how much Jesus had done for him (ὅσα ἐποίησεν αὐτῷ ὁ Ἰησοῦς). Thus, everyone there was amazed or marveled at this (καὶ πάντες ἐθαύμαζον). The Decapolis territory was a group of 10 gentile non-Jewish cities on the east bank of the Jordan River in present day Jordan and Syria that included the towns of Gerasa, Scythopolis, Hippos, Gadara, Pella, Philadelphia, Capitolias, Canatha, Raphana, and Damascus. Thus, this cured demoniac was the first Christian apostle to the gentiles.
There is something similar in Luke, chapter 8:38-39, while there is nothing like this in Matthew. Mark said that Jesus refused this former demoniac. He would not permit him to go with them (καὶ οὐκ ἀφῆκεν αὐτόν). But he said to him (ἀλλὰ λέγει αὐτῷ) to go his own home (Ὕπαγε εἰς τὸν οἶκόν σου πρὸς τοὺς σούς). Then he was to preach, announce, or tell others (καὶ ἀπάγγειλον αὐτοῖς) how much the Lord had done for him (ὅσα ὁ Κύριός σοι πεποίηκεν) with his great mercy for him (καὶ ἠλέησέν σε).