How to find Cloud cover data in GFS grib models
This is not an explicit answer but might give you an idea. The US National Weather Service forecasters use a tool called GFE to create gridded forecasts, largely starting from raw model data. For most models, even those that have TCDC as an output field, the model sky cover grids are computed by GFE using RH at various levels. For a few of the high resolution mesoscale models, the sky grids use the model cloud cover field and the resulting values are either 0% or 100%. Thus a blend of these differing sources can be used to get a more nuanced output forecast.