In objGravDown : Delete alarm 0 and 1. Change the collision with player into
if(global.reverse){
with(player){
playerReverse();
}}
In objGravUp : Same, delete alarm 0 and 1. Change the collision with player into
if(!global.reverse){
with(player){
playerReverse();
}}
In Sciprt playerMove : change the line 11 and 12 (or if you changed it, the yflag = -1)
yflag = -1;
L = keyboard_check_direct(global.leftbutton);
R = keyboard_check_direct(global.rightbutton);
There, idk if you already find it or not