Selling a Garage

Yes, you'll need a notaire as you will be transferring title deeds of a property.

Also, if the garage is attached to your property (or is included in your property i.e. a garage included with an apartment) you will need to split the two, and for that you may even need to get permission from the commune.

A notaire is probably your best stop for advice at least initially. You can use the same one you used when you bought your property.

The notaire nearest to Sq. Ambiorix is Notaire Jean Louis Brohée
https://www.notaire.be/Notaire/jean-brohee

Consider renting it, especially if it's attached to or comes with an existing property.