Here is a macro that will do this. You have to be on a chart sheet to run it, and you will have to change the path to suit.
Sub GifCht()
Dim FName As String
FName = Application.InputBox(prompt:="Please enter File name", Title:="Export chart", Type:=2)
ActiveChart.Export FileName:="E:Results" & FName & ".gif", FilterName:="GIF"
End Sub