The portal behind the gate is not disabled if the gate is undefeated. Video evidence: I tried leaping the castle of Sparta and encountered an invisible wall. Will try other levels and update if any are possible.

What are the mechanics for this skip? are you just hitting it at a precise point? is there a jump involved? i cant seem to hit it correctly haha. Thanks!

There is for sure a jump involved. The moment of impact on the stairs-bit (@22 - 23 seconds in the posted video) is a jump. As for hitting it correctly: The video attempt I am able to show are a 1 out of ~30 attempts to get over the wall. I do not know if the other runners have found a more consistent set up. For me: Jump into the stair-shaped wall and pray it works.

Ive just found a more consistent method that about 3 seconds slower which involves jumping repeatedly on the tall vases near the gate i recommend setting your jump key to the scroll wheel for this, its much more consistent than using the stairs although a little bit slower