Adding your own features is as simple as spinning up a new model, view and controller. Gravity comes with a CLI toolbelt that you can use to automatically generate the files needed for this.
gravity create-mvc viewName -db -ui
This will create:
a new model template in /model directory
a new controller template in the /controller directory that handles the server request and calls the appropriate model method
a new set of endpoints in /api.js to route the request to your controller methods
a React view in /client/src/views that makes the API request
If you don't want to create a database table, or a React view (if you're using Gravity Native for example) then you can omit the -db and -ui parameters.