As a matter of technical fact, it's not GPS time that gets adjusted with leap seconds, it's UTC. From a user perspective in most cases the difference isn't particularly meaningful because you probably want to convert between GPS time and UTC and for that use case it doesn't matter whether you add or subtract the offset to one parameter or the other. But the satellites don't update the time they broadcast every so often to align with UTC. They've been counting seconds as accurately as they can since they started broadcasting. Instead, they broadcast, in the GPS navigation message, the offset, in integer seconds, from UTC. If you are reading time directly from a GPS message, you never have to worry about it repeating or skipping an increment. UTC technically could do either one of those.

E: to be clear, the GPS control segment routinely updates the clocks on the satellites to maintain synchronization tight enough to meet the GPS specified error budget, but these adjustments are transparent to users and never anywhere close to entire seconds