Summarizing the gospel in my own words:
God, the Creator of the Universe, infinitely holy, desired to create man in His own image (Gen 1:26-27). He delighted to have man worship Him in the place He had prepared (Gen. 2:15), while providing for him commandments to live by (Gen. 2:16-17). In doing so, the prohibition to not eat from the tree of knowledge of good and evil and the promise of subsequent punishment by death for disobedience was given.
Man, though created sinless, still possessed the ability to sin and did so caving to the temptations set forth by the serpent, therefore rebelling against God by specifically disobeying the commandment given to him. Though the serpent first deceived Eve (Gen. 3:1), rightfully the blame fell to Adam (Gen. 3:9-12, 17) and it was in him that all mankind fell receiving in themselves the guilt incurred by him (Romans 5:12, 18). Subsequently, man was removed from the Garden and subjected to the curse of sin (Gen. 3:16-19, 22-24).
God, in His infinite grace and mercy while upholding the curse against man promised the blessing of a Redeemer by way of the Seed of the woman that would crush the head of the serpent (Gen. 3:15). This Seed, promised in this first gospel, has been revealed throughout redemptive history on the pages of Scripture to be none other than God Incarnate (John 1:14), the Son of God (John 1:34), our Lord Jesus Christ whom God the Father sent (John 3:16), conceived of the Holy Spirit (Matt. 1:20), born of a virgin (Matt. 1:23), to live sinless and blameless (2 Cor. 5:21; Heb. 7:26-27), and to be the propitiation for all who believe (Rom. 3:24).
Man, once alienated from God because of sin (Rom. 3:23) and dead in trespasses and sins (Eph. 2:1), could now be reconciled to God (Rom. 5:10) on the basis of the righteous fulfillment of the law by Christ (Heb. 8:4; Col. 2:14), His substitutionary death on the cross (Rom. 5:8), and His resurrection and His ascension to the right hand of the Father (Rom. 8:34) where He now rules and reigns as King (Acts 2:34-36), and mediates the new and better Covenant as our High Priest (Hebrews 8:1-6). Through repentance of sin and faith in the Lord Jesus Christ (Mark 1:15) by way of the regenerating work of the Spirit (John 1:13; John 3:5-8; Eph. 2:5), man has his relationship with God reconciled, is removed from under His wrath (Rom. 5:9), being now justified (Rom. 5:1), and having the righteousness that Christ has earned through His obedience to the law of God (Rom. 5:19) imputed to them (Rom. 4). Believers are therefore freed from sin (Rom. 8:1), having their sin imputed to Christ (2 Cor. 5:21; Gal. 3:13), and are subsequently freed to serve (Eph. 2:10; Heb. 9:14) and worship God (John 4:24; Rom. 12:1) as He intended. It is through grace alone, by faith alone, in Christ alone that all who believe are sealed by the Holy Spirit (Eph. 1:14), are to pursue a life of holiness and obedience (Eph. 4:22-24; Col. 1:10; Hebrews 12:14), while eagerly awaiting the return of Christ when all things will be consummated and reconciled in Him (Eph. 1:10; Rom. 8:21).