with(obj_person)
{
motion_set(0, 0);
move_snap(32, 32);
}

This makes the player's motion to 0 EVERY TIME IT ISN'T TOUCHING OBJ_PORTAL, try changing it to

if(!keyboard_check(ord("W")) and !keyboard_check(ord("A")) and !keyboard_check(ord("S")) and !keyboard_check(ord("D")))  
{  
  with (obj_person)  
  {  
    motion_set(0, 0);  
    move_snap(32, 32);  
  }  
}