The geometry of something like you want would be something really similar to what already exists in the 29er department. Chainstay will be longer like in a 29er and so. If anything, you could play a bit with reach (which in modern gravel frames is rapidly approaching that of a 2015-2020 29er), a bit lower stack (you can just run a slammed stem with maybe even more negative -17 deg angle to achieve that).
I don't know, that is what I would honestly do. You can just save yourself a lot of headaches. And particularly if running shimano you can just use PM mtb calipers and they should work just fine with the grx brifters.