Actually it does not matter what format Windly would use (GRIB or not) - it can be anything that transfers data economically from their servers to the app. To keep the file small, one should be able to choose only a number of variables, say wind (at ground level), gusts, waves, swell, etc. for a particular region and a defined timeframe (2 to 5 days ahead). This is what TimeZero provides when donwloading weather files.
I would like to second this request from a sailor's prespective.
It seems that ECMF produces weather front data already: https://apps.ecmwf.int/r/cdb/
It would very useful to see it in Windy.
I just want to second this request.
If you can make the Windy app to downlowad GRIB compressed files, that would be very useful for sailors. Even a slow satellite or 3G connection would allow download. I am not aware of any licening issues with the GRIB format.