Notice it says they shall be saved, not they shall get saved. They were already saved, the Gospel was sent to them to give them assurance and knowledge of their Salvation. When God sends saved people the Gospel, its to them the Gospel of their Salvation Eph 1:13
13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,
If Salvation wasnt already theirs, it wouldnt be "your salvation" The word your is hymōn and its a possessive pronoun and salvation is in the genitive case which usually is descriptive of possession,
So Cornelius was being given assurance of his Salvation, for God does that for them who are saved and fear Him,
Cornelius feared and and his house, we know that from Acts 10:1-2
here was a certain man in Caesarea called Cornelius, a centurion of the band called the Italian band,
2 A devout man, and one that feared God with all his house, which gave much alms to the people, and prayed to God alway.
Paul says that a word of Salvation is sent to them that fear God Acts 13;26
Men and brethren, children of the stock of Abraham, and whosoever among you feareth God, to you is the word of this salvation sent.
Because people who have the fear God have been saved, regenerated and given a new heart that fears God Jer 32:38-40
38 And they shall be my people, and I will be their God:
39 And I will give them one heart, and one way, that they may fear me for ever, for the good of them, and of their children after them:
40 And I will make an everlasting covenant with them, that I will not turn away from them, to do them good; but I will put my fear in their hearts, that they shall not depart from me.