System Down, Acid Rain, that works, but JUST against Gadgets. You have to prepare for other things, so why not mix up things that can counter Gadgets AND other stuff at the same time? And the biggest prob with Gadgets is not the field, but the HAND. You have to get rid of Gadgets in HAND. (Or negate the effect, i.e. Skill Drain)
By this stuff like Skill Drain also drops, (since it'll likely kill your strat as well)
Anti-Gadget cards IMO
Anti-Gadget Field Presence
Asura Priest
King Tiger Wanghu
Hydrogeddon
Royal Decree (For your monsters to actually kill Gadgets by result of battle)
Anti-Gadget Hand Disruption
Confiscation
Trap Dustshoot
Mind Crush
Don Zaloog
Spirit Reaper
Card Destruction
Morphing Jar
Chain Destruction (Seriously, this can kills Gadget outright. Destroys all 3x copy of either Green/Red/Yellow, and the whole search effect DIES. Like say your foe summoned Green Gadget and Chain Destruction is activated. They searched Red. And next turn, they summon Red to search Yellow. And that's it. All Yellow Gadgets are dead cards since there's no more Green Gadgets in your foe's deck. The only saving grace is Pot of Avarice. Of course, Chain Destruction is only good against Gadget... and thus not too recommended.)