The Gerber convention, like Blackwood, is a way of determining the number of aces when looking for a slam.
Gerber is only used when the partnership has agreed that the contract will be in no trumps.
After a bid of NT, a response of 4C asks for aces.
The response to 4C is:
4D = nil or 4 aces
4H = 1 ace
4S = 2 aces
4NT = 3 aces
After this round of bidding a bid of 5C may be used to ask for kings.
The response to 5C is:
5D = nil or 4 kings
5H = 1 king
5S = 2 kings
5NT = 3 kings
The advantage of using gerber with a NT contract, is that it is easy to stop the bidding short of a slam, if the number of aces is not sufficient.