It would be hugely ironic and strategic if the PLARF lobbed a nuke right on Pine Gap as a reply to a US tactical nuke attack on Chinese bases in the SCS.As of now since PLA at least officially don't have tactical nuclear weapons, its likely that they would just reply with a full sized nuke somewhere on US territory. If its just an island outpost that was nuked, then China probably would hit back on an American military base.
Ironic because Pine Gap probably has as many people as the Chinese SCS bases but is a massive nerve centre for all US drone and intelligence gathering SIGINT operations.