Ah, you want to replace unsemantic, not just add the 12 grid system within your content.
In that case, it’s a little harder, but do-able if you have some PHP knowledge.
2. You would use these filters (at the very bottom): https://generatepress.com/knowledgebase/filter-list/
With those you can remove/add classes as needed.
It might be easier just to use unsemantic for the overall layout, and use the 12 grid system within the site.