I actually have a new series on Youtube coming soon, showing how to use drag and drop to make a platformer. One of the videos covers one way platforms. I'm taking a different approach, and using traditional GML like hsp/vsp code to help make a later transition to GML easier, so even though it's for beginners, it's a fully featured platformer with some more complicated concepts.
The one way section is not quite ready, but generally you will want to create a separate platform solid and set that and your normal solid up as children to a parent solid object. Divide your collision check up into when you are going down and when you are going up. You then can check for different things to ensure you can move up through the one way platform, but collide with it when coming down.
I've only just uploaded the first episode, so check it out and subscribe so you will know when the one way platform episode goes live.