Here's what I'd do:
First, it depends on what kind of collisions you are using for your game. Whatever it is, just make the enemies collide with the door like it's a wall, but give no collision event for the player. I'm going to give a very simple example, because you're new to this. (I'm pretty new as well, only been coding in GML for a few weeks... It gets much easier when you get the hang of it.
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First, create a collision event in your enemy object. If you have multiple enemies, you can do it for all of them, or (this is what I recommend) you can do it for one enemy, and make it the parent of all the other enemies. Just be aware that making one object a parent of the others will make all your child objects take on the same behavior only for each event they don't have. So if your parent has a step event, all the code in the step event will be run by the children, but
only if they don't have their own step event. If the child has its own step event, it overwrites the parent. You can get around this by using the drag-n-drop function called 'call the parent event' in the event in which you want to use both the parent code and the child code. Anyways, for a very basic collision system, you could put in something like 'moveSpeed = 0' (or whatever variables you use to move the object) in the collision event with the wall, or use the drag-n-drop button for instance movement, set it to the square in the middle and speed to 0. There are certainly potential problems with this collision system, and I wouldn't recommend it for any sort of intermediate use. However, seeing as you're just starting out, it should work fine for your purposes. So if you have your collisions with walls set up (both your player and enemies should probably collide with walls), just do the same thing for the enemy colliding with the door. This way, they will hit it just like a wall. Now, for the simple part: do nothing. Just omit that particular collision event with the player, and the player should be able to walk right through the door. Hopefully that works for you. If not, I probably either badly misunderstood what you were asking, or I'm lacking very basic Game Maker knowledge.
Anyways, good luck!