Golden Bullet Bill is a normal bullet that can be defeated by doing pretty much anything to it as long as you dont jump in front of it otherwise you will die.
These Bullets come from a cannon thats fires them from the direction they are pointing at and where you are, so if your right in front of it and its aiming at you then it will fire but it also fires the oppisite side to where its aiming at you as well so when you jump over it it will fire from the other direction.
Another relative of it is the Banzai Bill which is much bigger then the Golden Bullet Bill so you can expect it to have a ridicolously bigger cannon then the Golden Bullet Bill.
Some Golden Bullet Bills is in new super mario 2 .will home in on you.
They can be pretty anoying.