I’d like to chime in with my support for this feature as well.
There’s no reason block elements can’t be used as a full-fledged mega menu builder. In fact, I think it’s a perfect pairing, and would be another compelling reason to choose GP for almost any project.
Astra, Blocksy and Kadence are already doing this.
At present, I would say that menu design is one of the weakest points of GeneratePress compared to popular alternatives and it’s one of the reasons I hesitate to recommend GP to non-devs, even with 2.0 Elements framework.
It could simply be added to the NavPlus module, and allow you to select a block template as a the menu item dropdown content from the official WordPress menu settings.