Concrete block wall below ground. Do they need to be filled. Concrete or can dirtor gravel be used

What type of wall is it? Retaining wall structure or is it part of a home and this is considered an "outside" wall. The reason for asking is because if it is as an "outside" wall then you may want to fill with insulation. If it is a retaining wall you could use rebar and cement to fill it for strength.

It will be used for a foundation for a garage

No they do not need to be filled.