@Override
  public void notifyTimestampsToGateways(EntryEventImpl event) {
    // Create updateTimeStampEvent from event.
    VersionTagHolder updateTimeStampEvent = new VersionTagHolder(event.getVersionTag());
    updateTimeStampEvent.setOperation(Operation.UPDATE_VERSION_STAMP);
    updateTimeStampEvent.setKeyInfo(event.getKeyInfo());
    updateTimeStampEvent.setGenerateCallbacks(false);
    updateTimeStampEvent.distributedMember = event.getDistributedMember();
    updateTimeStampEvent.setNewEventId(getSystem());

    if (event.getRegion() instanceof BucketRegion) {
      BucketRegion bucketRegion = (BucketRegion) event.getRegion();
      PartitionedRegion partitionedRegion = bucketRegion.getPartitionedRegion();
      updateTimeStampEvent.setRegion(partitionedRegion);

      // increment the tailKey for the event
      if (partitionedRegion.isParallelWanEnabled()) {
        bucketRegion.handleWANEvent(updateTimeStampEvent);
      }

      if (partitionedRegion.isInitialized()) {
        partitionedRegion.notifyGatewaySender(EnumListenerEvent.TIMESTAMP_UPDATE,
            updateTimeStampEvent);
      }
    } else {
      updateTimeStampEvent.setRegion(event.getRegion());
      notifyGatewaySender(EnumListenerEvent.TIMESTAMP_UPDATE, updateTimeStampEvent);
    }
  }
This notifies all WAN sites about updated timestamp on local site.
geode-core/src/main/java/org/apache/geode/internal/cache/LocalRegion.java