kim good luck!! this is some really cool progress. my quick guess might be that you could make a 2D array for the x,y coordinates past and then storing those and drawing them at different opacities as they go further down in the array :0c it might be a bit heavy though so maybe google around to see if there's anyone who's done trails in GML before.
omg awesome yess ?!?? i'm not too familiar with gm's path class but if you can get the coordinates you're golden!! you prob want something that refreshes the array every tick--moving each position further down and getting rid of them after x index. all of these are associated with a certain opacity inside the 2D array.