If your gardener is not willing to issue invoices, I suggest that you either
  1. Find yourself another gardener
  2. Contract him as a salaried worker
  3. Report him to the Tax Office
Having someone employed without a contract can turn into a real nigtmare. However, if you trust this person, you might want to take the risk of leaving things as they are, as many do in Spain.

As per the issue with your landlady, it's more or less the same story. Not many people declare the rental income in Spain. The consequences for the tenant are none, but if you are using the property for commercial purposes, you migh want to have an invoice to claim a refund on the VAT paid, and offset the costs in your tax return.