Gerber is an Ace-asking convention similar to Blackwood.
The difference is that Gerber uses 4♣ as the asking bid, rather than 4NT, which allows the bidding to be kept to a lower level.

When you have determined that the partnership has enough combined strength to be in a slam contract you may still be worried about the possibility that the opponents have 2 Aces out against you. The Gerber Convention allows you to ask partner how many Aces he holds.

A 4♣ bid by you asks partner to respond according to the following:
  • 4♦ : 0 or 4 Aces (if you can't tell which you shouldn't be asking)
  • 4♥ : 1 Ace
  • 4♠ : 2 Aces
  • 4NT : 3 Aces
If you find that your side holds all 4 of the Aces, you can then bid 5♣, asking for partner's Kings.
The responses will be similar, but at the 5-level.
  • 5♦ : 0 or 4 Kings (if you can't tell which you shouldn't be asking)
  • 5♥ : 1 King
  • 5♠ : 2 Kings
  • 5NT : 3 Kings
Gerber's advantage over Blackwood is that it keeps the bidding lower, yet provides the same information. The big problem with Gerber is that it is not always obvious whether the 4♣ bid is meant for Gerber or is being used in a different sense. Your partnership must have a strict agreement on just when 4♣ is Gerber asking. Our partnership uses this:

  If 4♣ is a jump bid immediately following partner's 1NT or 2NT call it is Gerber.
  Any other 4♣ bid is not Gerber.

BakerBridge has a topic called Gerber Quiz to help partnerships check their Gerber agreements.

Combined Blackwood / Gerber lesson available.