I will try to explain how to do health bars. But I haven't used GM in years so bare with me. Plus I suck at explaining things.
Also, This is only if you're not using views.
First off, make a new object and name it something like "health control". Next add a draw event to the object. Inside the draw event place the action "Draw health bar" (It's in the "score" section.)
This is the part I don't remember (so I may be wrong), you got to set the Xs and Ys. IIRC These are the coordinates of corners of the square that will be the bar. It tells where the bar will be drawn. To get the coordinates go to the room and use the grid to pick where you want corner 1,2,3, and 4.
I would help with the rest but I don't really remember much about GM.
Edit: Also if you wish to make a event that would cause the health bar to go down use the "set health" action and make sure to set it as relitive.