first when gun is on the ground you may use simple trigger.

if(event_type == event_trigger) && (you == player)
{my.passable = on; my.invisible = on; Player_have_this_gun =1;}

my.fire_power = 10;
while(me)
if(key_1 ==1) && (Player_have_this_gun ==1){my.invisible =off; player.fire_power = my.fire_power;}
if(my.invisible == off)
{
vec_set(my.x,player.x};//or use vec_for_vertex to attach to player's hand
my.pan = camera.pan; my.tilt = camera.tilt; //if FPP
if(mouse_left ==1){ fire();}//trace from muzzle to vec_for_screen, add effects etc. If result of trace !=0 and you == enemy then your life -= player.fire_power *time_setp; Just example
wait(1);
}