The GPS system doesn't recognise or use leap seconds. If you have an NTP device giving out correct UTC info from a GPS time reference it must also have a way to get leap second data from somewhere (presumably the Internet.)
Leap seconds can't be predicted, they result from measurement of the Earth's spin. Presumably a committee then issues a decree on vellum, or something. In theory leap seconds can be added or subtracted but so far they have all been added. Last time I looked UTC was about 12 seconds ahead of GPS but it may well be more now.
(Just checked - the committee is run by the International Earth Rotation Service. I presume they have a very large brass key to wind the old girl up when she is slowing down.)