I am on record saying that Gerber is a brand of baby food. I have also referred to Gerber as an infestation, an outbreak and a plague. Gerber is something my friend Harry Falk called a N.I.T.L.F.Y. convention, which stands for “Not In The Last Forty Years.” The reason for my aversion to Gerber is that people insist on treating Gerber as a way to explore for slam; that is not what Gerber does.
Gerber allows you to find out how many aces your partner has after an opening (or rebid) bid of 1NT or 2NT. Asking for aces (or keycards) is an incredibly important tool for slam bidding in a suit contract. In a suit contract, you are much more likely to want to play slam with 30 points or fewer than if you had been considering a notrump slam.
In notrump, you want to play in slam if you have a combined 33ish high card points. Why 33? For one, if you have 33, you can safely bet that you won’t be missing two aces. For another, barring a side source of tricks, you need that amount of strength to manage to take 12 tricks.
To find out if you have that strength, you don’t want to use Gerber. Gerber will just tell you about aces. Partner opens 1NT and you hold: 
AJ10 
K43 
K75 
AJ95. You’d like to be in slam if partner likes his hand. If partner has a bad hand, you don’t care how many aces he has. You should raise to 4NT, invitational/quantitative.
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PARTNER
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With this hand, partner would pass 4NT which is a good thing because you will have a little work to take even 10 tricks. If you had used Gerber, you would land in slam--after all, you have all of the aces.
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YOU
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PARTNER
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You have the same hand, but partner's is a little different:
Compare the first hand partner held with this one. Now partner has 16 and a good five-card suit, so he will accept the invitation and you have 12 tricks after knocking out the ace of diamonds. Missing an ace can be better than having all of them.
Another example. Partner opens 2NT. If you only worry about aces (and bid Gerber) you will wind up in slam with hands like this:
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YOU
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PARTNER
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Remember that when you use Gerber (or keycard blackwood) you are committing to bidding a slam if you are missing only one ace/keycard. Here, you would bid slam after Gerber and go down even if your opponents don’t start with the A and K of clubs.
You want to give partner the choice to accept with a hand like:
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YOU
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PARTNER
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This hand has the same 20 points, but partner will recognize that his good controls and nice looking spade suit make this a good 2NT (when the range is only 20 or 21, a good 20 can be the high end of the range). That means he would bid 6NT after 4NT.
I hope these examples help illustrate that asking for aces is not a substitute for inviting slam.
One more example; your partner opens 1NT and you hold: A105 A4 A732 A1062. If you wheel out Gerber, I will come and find you and take away every deck of cards in your home. You have two options--either bid 6NT yourself or bid a quantitative 4NT. Can partner like his hand without having any aces? Sure! If partner held K97 K6 KQJ54 KJ9, partner would have a great hand for slam assuming the 4NT bidder had the usual 16 or 17 points.
Quantitative bidding is the superior method to find out if you belong in slam for most balanced/notrump hands. Once in forty years, however, you might pick up a hand where all you care about is the number of aces partner holds. These hands tend to have one or zero aces, but a source of tricks. After a 2NT opening you hold: 86 K7 K5 KQ109732, you want to be in 4NT, 6NT or 7NT if your partner has 2, 3 or 4 Aces.
Partner responds (in standard Gerber) 4 showing 0 or 4 aces, 4 showing 1 ace, 4 showing 2 aces and 4NT showing 3 aces. If you want to sign off after partner's response, you can bid 4NT to play. 5 by the hand that bid 4 would ask for number of kings.
Another hand where you might use Gerber; partner opens 1NT and you hold K 109 KQ103 AKQ1052. You can use Gerber since if your partner shows three aces, you will want to be in 7NT, opposite two aces you will bid 6NT and if partner has only one ace, you will sign off in 4NT.
You don’t encounter those last two hand types frequently, even less often when your partner shows a strong, balanced hand. The rarity of those events makes Gerber a convention that's should rarely be used.