how do you use gantt charts and agile development ?

Gantt charts show the whole project schedule or at least that's what my boss needs… If using agile development model you take requeriments constantly using the next example:

Define -> build -> release, Define -> build -> release, Define -> build -> release … and so on

how do you manage when the manager of the company asks for a gantt chart with the duration that's going to take the whole development project ? it's impossible determine an end date when you know you will get more requeriments after every sprint or new modifications

how do you guys manage these cases ?