You should have something like this:

float velY = 0.0f;
float playerY = whatever;
const float jumpHeight = 20.0f; // you can and should adjust that

const float GRAVITY = 100.0f; // adjust this also

while(true){ // your main game loop

if(some kind of button is clicked (but not held down)){
   velY = -jumpHeight;
}

velY += GRAVITY * deltaTime;
playerY += velY * deltaTime;

// draw player using playerY coordinate

}

The thing about jumping in this type of game is that technically the graphic is going in the Y direction, but the collision checking should not, because in the game you aren't changing your position, rather jumping in the same position. I don't think GameMaker's gravity is going to help in this instance. Instead remember that you need to keep variables your x/y position on the ground, and then when jumping, you don't change the x/y position, except if the person moves while jumping. The object itself has to increase and decrease the y position so that the sprite changes, but the 'ground' position shouldn't change.

As far as the jumping itself, you can calculate it in different ways. I'd check some platformer tutorials, as the jumping works the same there, at least if you remember what I mention in the above paragraph.

The thing about jumping in this type of game is that technically the graphic is going in the Y direction, but the collision checking should not, because in the game you aren't changing your position, rather jumping in the same position. I don't think GameMaker's gravity is going to help in this instance. Instead remember that you need to keep variables your x/y position on the ground, and then when jumping, you don't change the x/y position, except if the person moves while jumping. The object itself has to increase and decrease the y position so that the sprite changes, but the 'ground' position shouldn't change.

As far as the jumping itself, you can calculate it in different ways. I'd check some platformer tutorials, as the jumping works the same there, at least if you remember what I mention in the above paragraph.

Not sure to what extent I agree or disagree here. What you are mentioning applies to games that have emulated depth (or 2.5d), where the Y value changes as the character moves deeper or shallower into the screen. If this is the case you have a point where the character must land at the correct Y position but to say that it will always be the same as the Y position that the character had jumped from might be incorrect. For example what if the player is holding up and right while jumping? The character should land a bit higher in Y (deeper into the screen) and to the right of where it had jumped from. Otherwise you get that uncomfortable linear jumping that we saw back in the 16 bit days.

Solution, use dual vectors. One for the characters level position and an offset that applies to movement based animations (such as jumping or a lunging attack). You would then apply your velocity of movement based on the offset, render and calculate collision versus the true position being the offset from the position but always bring the character back to the new position which depending on what your doing might be part position and offset or might return to the original position. So for example, jumping would have the characters level movements effect x&y or the position while controlling the characters jump through the Y's offset. The characters Y offset would be effected by the jumping velocity and gravity while the position would be effected by directional input. The idea would be to add then subtract to the offset's Y bringing it back to the position Y as calculated by the input. For something like a lunging or thrusting attack we might use the offset to propel the character forward then backward in the X axis bringing it back to the start X position.

Just FYI, about moving while jumping, that's exactly what I was talking about in my post, you know where I was talking about moving while jumping, that's what I was talking about. I guess I wasn't quite clear on that detail...

On the other hand, I'd say you(Dan Mayor) explained the solution better than I did, where I just better explained the problem :)

With some modifications for Game Maker, this code help me a lot. Thank you Edvinas, the jumping now works fine. Dan, this is a 16-bit style beat em up, and yes, now i need assign variables for the position in the ground for calculate colisions, enemy attacks, etc...

Thanks for your help guys.

With some modifications for Game Maker, this code help me a lot. Thank you Edvinas, the jumping now works fine. Dan, this is a 16-bit style beat em up, and yes, now i need assign variables for the position in the ground for calculate colisions, enemy attacks, etc...

Thanks for your help guys.

Just a word of advice. You are outgrowing GM and unless you have a specific reason for continuing to use it I'd recommend looking at something less cumbersome.

There we go again...