Gerber was invented in the 1930's and has been quite popular over the years, so it is likely that you will encounter it even if you do not play it. In modern bridge, most expert pairs do not use it and I will look at why in a moment. As always my recommendation is to first and foremost play system that you and your partner enjoy and feel comfortable with rather than what is optimal.
What is it?
Gerber is the bid of 4♣ to ask for Aces. The key idea is to not get to 6NT missing two Aces - a fairly sensible idea. It is most commonly used after a natural no trump bid, but some people like using it as a total replacement to blackwood and using it after suit agreements as well. If you want to use Gerber, my recommendation is to only use it after a natural no trump bid, and keep intact 4NT as your Ace or Keycard ask for suit contracts.
How does it work?
The responses are the same/similar to blackwood response. Remember, we are asking about Aces only.
4♣ = Gerber
Responses:
- 4♦ = 0 or 4 Aces
- 4♥ = 1 Ace
- 4♠ = 2 Aces
- 4NT = 3 Aces
Examples
Lets look at some examples
Example 1
Here you have decided to bid 6NT with your 12 points opposite partner's 20-22 opening. You checked for Aces and partner confirmed to have 2, so you are sure you aren't missing two Aces.
Example 2
Here 4♣ is Gerber since it follows after a natural No trump bid, which 1NT was. You showed your 1 Ace and partner bid 6NT.
Why do people use it - what is the advantage?
1. I think the main advantage - Gerber is at a lower level than 4NT as a substitute Ace ask. Keeping the auction low has advantages, for example the player can stop in the contract of 4NT if partner doesn't have enough Aces.
2. A lot of pairs play that the bid of 4NT after a natural no trump bid is quantitative. If 4NT is quantitative and not blackwood, there is no convention to ask for Aces, which is where Gerber comes in.
What are the disadvantages?
1. The disadvantage is that the bid of 4♣ is used up, and cannot have natural meaning, for example. If partner bids a natural 3NT in the course of the auction, and you want to bid a natural 4♣, you cannot if you are playing Gerber.
2. Over 1NT opening, the bid of 4♣ has typically been assigned a different meaning by expert circles. Many players play it as Texas Transfers.
3. It can cause some confusion if there are any exceptions as to when 4♣ is Gerber, especially if played over a suit. A player may try to splinter but partner thinks it is Gerber. This can be largely resolved by partnership discussion and only playing it after a natural NT.
Overall recommendation
I do not recommend going out of your way to put this into your system, play it if it particularly appeals to you or partner, but otherwise give it a miss. It is not very common that partnerships get to slam, including 6NT missing two Aces. And anyway, there are other things to worry about as well, like missing the Ace and King of a suit (that is two tricks immediately for the defence in 6NT). Gerber might remedy some of the issues, but not all of them. Hand evaluation and good partnership bidding judgment is a more important skill than this convention overall. Specifically, both players being aware that Aces and Kings are important cards to have when thinking of Slam, and being careful without them - that by default will stop you short of slam when your hands don't have the correct cards.
Read next: Quantitative 4NT: How to Invite Slam in No Trumps · Roman Key Card Blackwood (RKCB) and Blackwood · Texas Transfers: Transfer Directly to Game