Pull the custom HTML templates with an initContainer into a emptyDir volume directory, for example /theme. The initContainer spec could look like:

      initContainers:
      - name: download-theme
        image: alpine/git:1.0.7
        command:
         - git
         - clone
         - "https://github.com/kinvolk/gangway-theme.git"
         - /theme
        volumeMounts:
        - name: theme
          mountPath: /theme/

Mount that emptyDir volume into your gangway container at /theme and point to the customHTMLTemplatesDir config parameter to it:

customHTMLTemplatesDir: "/theme"