Within the GLS Netherlands settings page in Shiptheory, it is possible to turn on a setting to allow customers to be notified of shipments being sent out.

To toggle this option, firstly navigate to your GLS Netherlands page in Shiptheory and scroll down to "Advanced Settings": 

If you scroll down to "Notification Emails" you can then turn this on or off, and customise some of the details with what's sent:

In terms of what's actually sent, there will be one email sent, and this will be sent when the order is booked with GLS Netherlands. It will contain a track and trace link.

Within this email, it's also possible to change the delivery date or delivery address.