I want to re-iterate that just because NVIDIA supports GBM doesn't mean wlroots will officially support NVIDIA.
I welcome NVIDIA's move of implementing the standard APIs, it's a nice step forward. However, it's still closed source. That means not only developers are forced to install closed source software on their machine, but also that they work with a black box.
We're routinely sending patches to upstream and downstream projects like the kernel drivers, Mesa, GTK, Firefox, etc. This is only possible because they're open-source software.