"It's over," said Gu Qingcheng. After Han Sen and Ancient Devil got sucked into God's Door, her face changed horrifically.
God's Door was the path by which demi-gods could reach the Fifth Sanctuary. It was the path creatures took. The power inside would crush the person, rebuild their body, and rinse them clean.
It was built for the person that summoned the door. The rinsing and rebuilding would have to match with the element.
A person who had an attunement to a fire element would be washed and rebuilt with fire, once beyond God's Door. Fire casters have the advantage that would allow them to finish the process and be rebuilt successfully.
If a water element person entered the fire element God's Door, they would be rinsed and rebuilt with fire just the same. With a lack of proficiency with the element, though, they would not succeed. That was why Gu Qingcheng was in such shock, after seeing Han Sen get sucked inside.