First, notice that by Paul saying, "which is not another" and "any other gospel," it is clear that there is only one true Gospel. That is extremely important. In his second letter to the Church at Corinth, he wrote, "But I fear, lest by any means, as the serpent beguiled Eve through his subtilty, so your minds should be corrupted from the simplicity that is in Christ. For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him" (2 Cor. 11:3-4).
Lest the last phrase be interpreted as Paul suggesting that they tolerate the messenger of Satan with a false gospel, the context indicates Paul is being sarcastic. Other translations show this: the NIV has "you put up with it easily enough." The ESV is "you put up with it readily enough." The NASB reads, "you bear this beautifully." And the RSV has "you submit readily enough." Paul is actually protesting that they were quick to accept false teachers, but resisted the truth from him (see verses 13-15).
Although there are many instances where the true Gospel is said to be "of Jesus," "of Christ," "of grace," "of peace," "of God," the clearest indication that there is only one Gospel is found in the using of the definite article, "the." Of the 101 times the word appears in the Bible, only twelve do not have the definite article; they are: "my Gospel" (Rom. 2:16; 16:25; 2 Tim. 2:8); "Christ's Gospel" (2 Cor. 2:12); "our Gospel" (2 Cor. 4:3; 1 Th. 1:5; 2 Th. 2:14); "another gospel" (2 Cor. 11:7; Gal. 1:6, 7, 8, 9).
So what is the true Gospel? It is the good news that God sent His Son to pay for our sins (Jn. 3:16). That He was crucified, buried, and rose again according to the Scriptures (1 Cor. 15:1-4). And by faith in Him, we are forgiven of our sins (Eph. 1:7), and we become the adopted children of God (Rom. 8:15-17)! Now that is the Gospel truth!