The liscencing issue only applies to ECMWF. All model output produced by USA agencies is public domain (free to use, no liscencing). So Windy could work with GRIB output from GFS with no issues. Windy already supports use of GFS so this should be a relatively straight forward implementation.
Implementing GRIB viewing capability in Windy would be a powerful addition. End users could download their own GRIBs, from whatever model(s) they choose, and view in Windy (as is now done by all other GRIB viewers).
For us offshore sailors, who commonly have the ability to download a GRIB over low bandwidth interfaces which wont reliably support an online session, this would be a great upgrade vs existing GRIB viewers (many of which are a bit clunky).