Gnosticism was a large umbrella of varying perspectives around basic beliefs heretical to the Christian faith. Gnosticism tended to be based on dualism, this idea of good vs. evil, material vs. spiritual. Gnostics were always on a search for "hidden knowledge" that would bring about salvation, which is in stark contrast to the Christian faith, that indicates faith in Christ as the means to salvation. Generally in gnosticism the idea is that the physical world is evil, the physical world is bad, the human body is bad, and things spiritual are good.
In fact, gnostics believed that the natural world itself was the prideful creation of an evil false god. Christ was the solution to humans escaping from the material universe. So Christ can't be physical, because the physical is evil, so Christ is seen as a ghost. Christ also isn't considered to be God either, just one of many spiritual beings.
The early church father Irenaeus dealt with the heresy of gnosticism two thousand years ago. He challenged the gnostics, not by individually dealing with all the various sects of gnosticism, of which there many, but instead by simply addressing the main ideological viewpoints of the gnostics, and refuting them. To his mind, this was as good as refuting all the sects at once.
Irenaeus regarded creation documented in Genesis to be fundamentally a good thing. Humanity and the universe were created good, which he would indicate is counter to what the gnostics taught, which was that creation was fundamentally bad. Irenaeus validates the doctrine of the Trinity, which of course refutes the concept that Christ is a mere spiritual being and not the son of God.
Thankfully, gnosticism did not prevail over the ages. Though it still exists in some minor forms in the modern world. But how might things have been different if the early church lost out?
What would it be like in modern Christianity if gnosticism had prevailed? Well, since gnosticism essentially completely departed from the scriptures, church authority, and biblical theology, I don't suppose there would be any sort of Christian movement remaining in the world, unless it picked up again after the imminent collapse of the victorious gnostic movement. Any church movement that charges in the direction of lowered view of scripture, heresies, and neglect for church authority tends to disappear over time. At least that has been the case in more recent times with church movements that begin to depart from the authority of scripture. Though one might cite the flourishing of heretical movements like the Jehovah's Witnesses and the Mormon church to counter that idea. In that case, I would suggest perhaps gnosticism would've morphed over the centuries, as such open-ended ideologies tend to, into a conglomeration of religions something similar to Unitarian universalism. That's just a guess, but it's certainly plausible.
