Jumping in a top-down game is tricker. You’re already using up the X and Y axes for 4-directional movement, so for jumping, you now need a third axis: Z.
The same jumping mechanics that you apply to the Y axis in a platformer game, can be applied to the Z axis, in a top-down game. To store the value of that axis, you’d have to create a variable (z).
Then, when drawing that instance, you’d have to add z to the Y-axis:
draw_sprite_ext(sprite_index, image_index, x, y+z, ... );
Ground Collisions
For checking whether you’re on the ground, you can simply check if z is greater than 0.
Links
See this tutorial: Z-Axis in 2D Games
For other topics on jumping, see this: How to make the player jump?
Similar: