Here are some statements of beliefs from some evangelical fellowships: The Gospel Coalition, IFES, Wycliffe.
Evangelicalism is a branch of protestantism, so it involves a protestant understanding of the gospel. But not all protestants are evangelical.
I think the biggest distinctive is what they say about the Bible. They reject those who say the Bible is flawed in some way, not relevant for today, that you need the authority of church leaders to understand it, or that God gives new relevations today that take priority over the Bible. To quote from the Gospel Coalition:
Revelation God has graciously disclosed his existence and power in the created order, and has supremely revealed himself to fallen human beings in the person of his Son, the incarnate Word. Moreover, this God is a speaking God who by his Spirit has graciously disclosed himself in human words: we believe that God has inspired the words preserved in the Scriptures, the sixty-six books of the Old and New Testaments, which are both record and means of his saving work in the world. These writings alone constitute the verbally inspired Word of God, which is utterly authoritative and without error in the original writings, complete in its revelation of his will for salvation, sufficient for all that God requires us to believe and do, and final in its authority over every domain of knowledge to which it speaks. We confess that both our finitude and our sinfulness preclude the possibility of knowing God’s truth exhaustively, but we affirm that, enlightened by the Spirit of God, we can know God’s revealed truth truly. The Bible is to be believed, as God’s instruction, in all that it teaches; obeyed, as God’s command, in all that it requires; and trusted, as God’s pledge, in all that it promises. As God’s people hear, believe, and do the Word, they are equipped as disciples of Christ and witnesses to the gospel.
Why is the movement called evangelicalism when it's primary distinctive isn't about the gospel? I think because historically the people they were reacting against neglected the gospel because they neglected the Bible. Promoting the authority of the Bible is a safeguard to ensure the gospel keeps first importance.