Here I'll provide a basic code for you to use. And put a couple of things to note about the variable.

Before we start coding we need to set it in init.

//

stage_freefalling = false;

//

Then you can copy and or edit this piece of code.

//
if stage_freefalling && state != PS_ATTACK_AIR && state != PS_DOUBLE_JUMP && free && vsp > 0
{
sprite_index = sprite_get("yourspritehere");
image_index = get_gameplay_time()/6;
}
//

Use this code as a starting point for whatever you want to create for free fall.

Note that the variable doesn't check for your vsp.