This can be done via the web application's Drag-Drop Editor.
1. Go to the web application to create your template: https://app.gmergeplus.com/template
2. Once you have finished adding content to your template click on Save in the top right hand corner
3. In the window that pops up type the Category name on the line Categories
4. Press Enter/Return on your keyboard. Once the Category name has turned grey and is saved you will be able to click SUBMIT to save your new template and category
5. You will be then able to select your template under the Categories section of G Merge Plus Mail Merge