I hope you are well today and thank you for your question.
Those posts are automatically displayed there related to the post and you can’t control which specific category posts to display there.
To achieve this, you have to develop custom code in the child theme of the Gillion theme.
Developing custom code for custom functionality is beyond the scope of support that we provide here. Please see https://themeforest.net/item/gillion-multiconcept-blogmagazine-theme/19470306/support
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