As I understand it Gnostic's were originally a group of christians that basicly believed that it didnt matter if the things in the bible were true or not, it was still good advice and should be followed.
No. Far from it. The Gnostic-Unitarian-Trinitarian division predates the Vulgate. There were many conflicting Gospels, as everyone wanted their bit of influence over Christianity, and many decidedly non-Christian teachings made it into the Bible e.g. the breaking of the bread.
Gnosticism teaches that the material world is a false, imperfect thing, and created by a flawed entity known as the Demiurge. In many original Gnostic sects, the Demiurge was the God of the Hebrews - Yahweh. In others, especially later versions such as the Cathars, the Demiurge was Satan. The Demiurge is sometimes depicted as evil, other times, merely flawed. Where the Demiurge was Yahweh, the Hebrew scriptures are thought of as somewhat true - but from a different perspective - Elohim is afraid
of humanity, and seeks to keep humans from achieving Gnosis (knowledge) and thus surpassing him. The Serpent - Jesus - comes to Earth to attempt to give humans this special knowledge, that they may escape from the Demiurge's prison.
Because of this belief - that the material world is a false, illusory thing - Gnostics did not believe that Jesus was human. Which is what this line
For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist.
Same with 1 John 4
Beloved, believe not every spirit, but try the spirits whether they are of God: because many false prophets are gone out into the world.
Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:
And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
These passages specifically attack the Gnostic belief that Jesus was not of the flesh.
Likewise, the passage from 1 John 2
Who is a liar but he that denieth that Jesus is the Christ?
An attack on Judaism
He is antichrist, that denieth the Father and the Son.
Whosoever denieth the Son, the same hath not the Father: he that acknowledgeth the Son hath the Father also.
An attack on Unitarianism. From this, Muhammad was also an antichrist.
The entire concept of the antichrist was as an attack on people professing versions of Christianity that the authors did not like.
In christian theology, and lets be serious that really what were talking about here. The anti-christ is depicted as a wholly different person. In the bible (I believe John 1 and 2),
Christian theology has more to it than you think. And see above.
The concept of the anti-Christ is that it is a person who either puts up a 'wrong' depiction of Christ in his place - an illusion or lie. Pauline Christianity has no monopoly on the truth in this regard, they just murdered most of the others. "Kill them all, God will know his own" comes from the persecution of the Cathars.
Nostradamus has made no verifiable predictions. People only claim he predicted something after the fact. That does not qualify.
and I believe also some Jewish and Islamic texts its the same.
The Masih ad-Dajjal bears some resemblance to the Man of Sin and the False Prophet, yes, but I'm not aware of any such in Judaism.
I do not know of any actual written document discussing Jesus being the anti-christ within actual christian Theology so I doubt the legitimacy of that claim
Modern Christian eschatology is no more relevant than any other work of fiction. I find early Christian history fascinating, personally, but there's nothing supernatural to it - it was nothing more than political mudslinging of the time.