Thank you for your question.
Yes it is related to category.
To change it to related to tags, you have to develop custom code in the child theme of Gillion theme.
Developing custom code for custom functionality is beyond the scope of support that we provide here.
If you are not a developer then you can consider hiring a developer to develop it for you. You can hire a developer from any freelance site. Shufflehound recommends the developer https://www.upwork.com/freelancers/~011652ffec8865c6d5