I've explained this a few times in the last weeks, so forgive me for being brief. I refer you to one of a few million (that's an exageration if you didn't catch that) active topics on gadgets going on right now.

Gadgets, when summoned, bring another gadget to your hand. That does two things for you :

1.You get the equivalent of two draws per turn to their one.
2.Regardless of how many monsters you summon, you'll always have a monster to summon in your hand.

The point of a fifth gadget deck is to exploit this. Turn your card advantage into a win condition by simplifying the duel. If you keep making 141 trades, you can bring your opponent to a topdecking situation, while you have a host of options and at least one monster left to play. You'll especially want to target monsters. Unless its a burn deck, that keeps them from dealing damage, and secondly, with spells and traps, if you play correctly, all they can do is make 141 trades, which is in your best interest since it simplifies the duel.

In short, your tiny gadgets will be attacking directly, a LOT of times. Combining for a rather easy win.

If you happen to draw Ultimate offering or limiter removal, you can often finish the game in just a few turns as well, but gadgets have the control factor that allows you to play the duel as much as you want.

Overload fusion, since you asked, is not a good card for a typical fifth gadget deck, because you'll want to use pot of avarice. Maybe twice if you can time magician of faith. You don't want anything removing those gadgets. You CAN build a gadget overload deck, but usually that will center around exploiting gadgets to get your hands on trunade or storm abd some monster removal.