If you block days in the diary they will now show as a different colour in GG, as shown:-
The colour used for blocked days is defined by the CACO parameter as below, the blocked colour shown above is B00000.
CACO D1E3F8 800000 B00000
If CACO does not have a 3rd parameter value they will appear in black.
Blocked days in the diary are only shown in GG and not through the external interface to guests, here they will appear as booked.