To set up a Gas Service Reminder without having issued any certificates, first you'll need to add the customer and job address if you haven't done this already:
Next you'll need to add a job address for this customer:
You can set the service reminder from this page:
If you want to add this for an existing customer, you will need to find the page on which you can edit the job address, which is almost identical to the image above.
You can find this by searching for the customer or job address in the search bar, or navigating to the job address using the Job Address tab.