Gregtech Community Edition tools use NBT and a special "material" registry for its tools. This mod currently doesn't have any way to edit them. Can we have them be configurable as well?

IIRC GTCE already has a custom configuration system based to configure the harvest level of tools.

Given that and the lack of time and motivation I currently have for this mod, it probably won't happen soon, if at all.

You actually can't. Pixel brought the issue up in their github, but it was closed with prejudice.
Oh well, I understand the lack of motivation. Still hope it'll be a thing one day, though.