// filter for year for GPP and NPP
 var GPPy = ee.Image(gppCollection.filter(ee.Filter.calendarRange(y, y, 'year')).sum());
 var NPPy = ee.Image(nppCollection.filter(ee.Filter.calendarRange(y, y, 'year')).mean());
 var npp8 = myimg.expression('(GGP8 / GPPy) * NPPy',
{
    GGP8: myimg,
    GPPy: GPPy,
    NPPy: NPPy
});

 // multiply with scale factor
 npp8 = npp8.multiply(0.0001);