Customizing BlueCat Gateway usually means creating new GUI or REST API endpoints. In both cases you can get started through the Create workflow page (see Setting up a new Custom Workflow).
In general, you'll go through the following steps when building a new workflow.
Creating a workflow with a graphical user interface (GUI):
Create a skeleton for the workflow using the Create workflow page, selecting Workflow with UI as the workflow type.
For more details, see Setting up a new Custom Workflow.
Edit the HTML template for the workflow.
Edit the CSS template if needed.
Edit the JavaScript source file if needed.
Write the Python code that implements the workflow handler.
Creating a workflow with just a REST API:
Create a new skeleton using Create Workflow, selecting Workflow using only REST API as the workflow type.
For more details, see Setting up a new Custom Workflow.
Write the Python code that implements the workflow handler.