All-in-One Video Gallery plugin WP
Please check all these point for the Gallery Plugin. Make sure it's enabled for Gallery post type.
If the default comment system is also not available on this plugin you should contact them and ask them provide some support to enable comments on Gallery Pages.
I'm sorry but I didn't follow you. And you didn't answered on my question. Do you see the default comment form when the wpDiscuz plugin is deactivated? You you don't then contact them and ask them enable comment system in their plugin.
Also, have you checked the documentation point I mentioned above? is the Gallery Post Type enabled in Comments > Forms > Default Form settings? It's mentioned in the first point of the documentation: https://wpdiscuz.com/docs/wpdiscuz-documentation/getting-started/missing-comment-form/