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
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
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.