Apart from the one true gospel, no person can be saved. God grants us grace during the time the one true gospel is preached, whether by a minister of the gospel or Him. God is able to preach the gospel to one He has chosen. Read about the conversion of Paul.
To answer this question, “must I know and obey the “one true gospel” to be saved,” the one true gospel must be declared. The one and only true gospel is the Mashiach, Yeshua of Nazareth, died for our sins according to the Old Testament, He was buried and He rose again the third day according to the Tanach. (Luke 24, 1 Cor 15:1-6) Now that the one true gospel is settled, I can go on and answer the question. The question is meaningless if the gospel preached is not the one true gospel. If the gospel preached is not the one true gospel, it is no gospel at all.
If the gospel is preach to a person and God grants grace for him or her to accept it, he or she knows the gospel. It is accepted. To obey the gospel is not to live according to law, the letter of the law, but to live by grace and the Spirit. “Keep the commandment” is the exhortation. Or in the Spirit, “keep the prophecy”. Do you not know that all the Law, the Torah, is prophecy concerning the Mashiach? “The Prophets and the Law prophesy until Yohan.” The Torah first and foremost is prophecy, the foretelling of the Mashiach who is Yeshua of Nazareth. So to answer the first part of the question “must I know?” Of course. God must give you grace to know the one true gospel or no person can be saved. Keep the Law lawfully and remembered the prophecies by the grace given you. This is the lawful keeping of the Law!
And how does one obey the gospel? By continuing in the grace and continuing to attach oneself to this form of teaching. How does one continue in the grace of God? By God continually giving grace. It is grace at the beginning, and grace to the end. And what is the form of teaching? The one true gospel I know I preach is the oly form of teaching God has to teach His chosen ones. If you continue to walk in grace attached to the one true gospel, you will truly be the disciple of the Lord and you shall be saved. “If you abide in My word, then you are truly my disciples.” They who endure to the end shall be saved. They shall be saved from sin, death and judgment.
With obedience there is dominion and authority and the preaching of the one true gospel, the preaching of the Mashiach. I agree with Paul, “Not that we have dominion over your faith, but we are helpers of your joy. For by the faith you stand.” And “the faith” is the “faith of Yeshua”. There are not many faiths, but one faith. And this is the faith of the one true gospel, I am sure I preach. God is my witness.