What it does
Gigaton Hammer is a Steel-type physical move with very high base power and 100% accuracy. The only constraint: after you use it, you cannot select it again the immediately following turn. Not a two-turn recharge, not a cooldown that locks you in — just 'not this move again next turn.' One turn later, it's available again.
Steel-type moves hit Ice, Rock, and Fairy super-effectively. Fire and Water resist Steel; other Steel-types resist it too. The 100% accuracy is reliable, no misses to account for.
In doubles, the 'can't use twice in a row' restriction is less limiting than it sounds. You're typically not burning two consecutive turns on the same Pokémon anyway — your partner is doing things on alternating turns, and you can rotate to Play Rough or another move on the off-turn. The rhythm becomes natural quickly.
Who runs it
Tinkaton at 96% usage — this is its signature move. Tinkaton is Fairy/Steel-type, and Gigaton Hammer provides Steel STAB alongside its STAB Play Rough. The two together give it coverage against Ice, Rock, Fairy (Steel), Dragon, Fighting, and Dark — a very wide offensive footprint for a single Pokémon.
With Choice Band, Gigaton Hammer outputs damage that KOs a huge portion of the unboosted metagame in a single hit. Even without the band, the raw power turns it into a reliable two-hit finisher on most defensive targets.
How to use it
The natural rhythm: Gigaton Hammer this turn, Play Rough or a support move next turn, Gigaton Hammer again the turn after. In doubles this slots cleanly into the flow because your partner is attacking the other target on the 'off' turn anyway — nothing is wasted.
If you need to finish a weakened target over two turns: Gigaton Hammer for the big chunk, anything else to finish, then Gigaton Hammer again if needed. The one-turn gap is workable.
Against fast threats that might KO Tinkaton before it can move, pairing with a Fake Out user (someone like Incineroar) to lock the threat down lets Tinkaton swing first.
Tips & strategy
Fire-types halve Steel damage — don't point Gigaton Hammer at them. Play Rough handles Fire-types adequately (neutral, not super-effective, but the raw power still does work). Water-types resist it too.
Gigaton Hammer is a contact move, which means Rocky Helmet users reflect chip damage back at Tinkaton. Factor that into HP management; if the opponent has a known Rocky Helmet carrier, consider letting your partner handle it.
Don't try to use Gigaton Hammer two turns in a row — it simply won't be selectable. If you try to click it when it's blocked and get redirected to another move, that's a preventable mistake. Have the alternate move plan ready before you sit down.