What's a good counter-strategy to Workshop/Garden?
Workshop/Gardens is usually a dominant strategy. There are a couple of exceptions:


Bishop can counter Gardens by buying Gardens and then trashing them for VP. This prevents the Gardens player from being able to get a favorable Gardens split and means that the Bishop player can focus on getting VP chips or Duchies while the Gardens player is buying out Workshops and whatever other pile.
Cursing attacks can often foil a Gardens strategy. Since Gardens games tend to end fast, a difference of 3 or 4 points can be huge. The threat of pumping up the value of the opponent's Gardens is overrated; even if the Gardens player gets all 8 Gardens, 10 curses would still be a net loss of 2 points.
Discard attacks can significantly slow a Gardens player. Only having 3 cards in hand means that the person can probably keep a Workshop and 2 treasures, which will frequently be Coppers. That's not enough money to keep acquiring Workshops or to get a double Gardens. Ghost Ship is especially brutal because the Gardens player will have lots of bad cards, and so forcing him or her to see those cards multiple times instead of being able to just discard them is really painful.
Embargo can make Gardens unattractive very quickly. Put an Embargo on the Workshop pile, then the Estate pile, then the Copper pile. Embargoing the Gardens themselves is ok, but usually the gardener will be gaining Gardens via Workshop and so the Embargo won't matter. If you Embargo their buy possibilities, however, it quickly slows down their deck.
A quick mega-turn engine may be able to overwhelm a Gardens deck. I'm thinking about Native Village/Bridge, which can be used to end the game in a single turn. This engine might be fast enough to buy a couple of Provinces, drain the Estates, and end the game on piles. It's ridiculously difficult to simulate because of the pile-ending conditions and needing some intuition in order to decide when to pull the mega-turn, but I played a couple of solo games earlier today and it seems viable as a strategy.