Hugo Version 0.31.1 on Gitlab

You have 2 options.

  1. Use a different Hugo Docker image on Gitlab that is more recent.

  2. Choose the Hugo version you need on Gitlab.

EDIT
Upon closer inspection of your .gitlab-ci.yml you are using the latest Hugo at the Gitlab container registry and that is version 0.30.2

Hugo 0.31.1 does not exist in the Gitlab container registry yet: https://gitlab.com/pages/hugo/container_registry

The hugo/monachus image is also v. 0.30.2 v. 0.31.1 see here