Or if the SST grid is modulo in x (SHO GRID SST), then try SST[x=0:361@fnr] and specify FILL/x=0:361.
yes? set data coads_climatologyyes? fill/l=1 SSTyes? fill/hlimits=0:360/l=1 SSTyes? fill/hlimits=0:360/x=-1:361/l=1 SST