While Gadgets can miss their timing, if Giant Rat special summons one (and it is the last thing to occur in the chain), you will get the effect.
Example: Heather attacks Simon's face down with Jinzo. The face down is Giant Rat. Giant Rat is destroyed and Simon chooses Yellow Gadget to Special Summon to the field. That effect would activate and he can add Green Gadget to his hand.
BUT
Example: Heather attacks Simon's Giant Rat with Mystic Tomato. Since both monsters are destroyed, their effects go on a chain: Heather's Tomato (as Turn Player) is Link 1, Simon's Rat (as Non Turn Player) is Link 2. Simon Special Summons Red Gadget to the field. However, since the chain must resolve first, Heather Special Summons Snipe Hunter to the field. Because the Gadget being summoned was not the last thing to occur, you miss the timing in this instance.