Gen. 2:16,17 is not gospel; it's law.Well , we do know where Gen 3:15 is going but I believe the gospel was preached to Adam in Gen 2 , the gospel main message has not changed when you really think about it , Obey God and live . That would be the message to pass on .
Gen 2
16And the LORD God commanded the man, saying, Of every tree of the garden thou mayest freely eat:
17But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
The gospel is the good news of Jesus' victory over sin, death and Satan, by his shed blood and substitutionary death on the cross, and his bodily resurrection. This good news was not declared (even in predictive prophecy, like Gen. 3:15) until the law had been broken.
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