The dangers of Gnosticism in the 1st century, were that it perverted the true teachings of Jesus Christ. The people were being led astray and took all the value out of the sacrifice of Jesus in the cross as not being sufficient for salvation. The false teachers taught that mere humans had a lot more to do with their own salvation, and not rely solely on Christ. Gnosticism also denied that Jesus was fully human-God.
The dangers of Gnosticism are still prevalent today, I think in more varied forms. The false teachings are still having an impact on the believe of the true Gospel. People are more confused today, because all other teachings seem to be a good alternative to Christianity. The danger comes by the fact that the second coming of the Lord is nearer today than it was in the first century. Another issue is that back then they had the apostles performing all these signs and wonders, and that proved on whom the power of God rested in, today we don’t see many believers walking in such authority. So maybe is more dangerous today than it was back then, there is still a lot of opposition to the Gospel and not enough believers confronting the heresies of our day.