You could define a "derived" property mapped as a formula to extract the date part of the date-and-time. The exact formula will differ depending what DB you're using, for MySQL you could use something like
Date creationDay // not sure exactly what type this needs to be, it may need
// to be java.sql.Date instead of java.util.Date
static mapping = {
creationDay formula: 'DATE(creation_date)'
}
(the formula uses DB column names rather than GORM property names). Now you can group by creationDay
instead of by creationDate
and it should do what you need.
Or instead of a "date" you could use separate fields for year, month and day as suggested in the other answer, and I think those functions are valid in H2 as well as MySQL.