By default, a default invoice template is provided. You can use it, but you are not allowed to change it with Gestinux.
You can customize it, adding for example one or more logos, and changing the place and aspect of fields and labels. To do this you can start with the template provided, and store a modified copy of it.
If you have several templates, you can define the template to use with a sale category, with a customer or for each invoice if you want.
Modifying a layout uses the Lazarus report designer, a fork of FastReport. It requires some knowledge to be used, but you can ask help from a developer to customize your documents.