1 Corinthians 15:
 Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand;
 By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain.
 For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures;
 And that he was buried, and that he rose again the third day according to the scriptures:
The Gospel of the Grace of God is a past tense gospel, so it could not have been preached before it actually happened.
Perhaps Paul meant what he said....
"But I certify you, brethren, that the gospel which was preached of me is not after man. For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ." (Gal. 1:11-12)
"Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began," (Rom. 16:25)
"But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed. Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith." (Gal. 3:23-24)
And perhaps the following verses mean what they say as well...
 From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
 Then Peter took him, and began to rebuke him, saying, Be it far from thee, Lord: this shall not be unto thee.
 But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men.
 And as they came down from the mountain, he charged them that they should tell no man what things they had seen, till the Son of man were risen from the dead.
 And they kept that saying with themselves, questioning one with another what the rising from the dead should mean.
 For he taught his disciples, and said unto them, The Son of man is delivered into the hands of men, and they shall kill him; and after that he is killed, he shall rise the third day.
 But they understood not that saying, and were afraid to ask him.
 Let these sayings sink down into your ears: for the Son of man shall be delivered into the hands of men.
 But they understood not this saying, and it was hid from them, that they perceived it not: and they feared to ask him of that saying.
 Then he took unto him the twelve, and said unto them, Behold, we go up to Jerusalem, and all things that are written by the prophets concerning the Son of man shall be accomplished.
 For he shall be delivered unto the Gentiles, and shall be mocked, and spitefully entreated, and spitted on:
 And they shall scourge him, and put him to death: and the third day he shall rise again.
 And they understood none of these things: and this saying was hid from them, neither knew they the things which were spoken.