Gyproc Fireline, Knauf Fireboard and Glasrock Fireboard can all be painted, so, if you have any of these products, you can paint them. Other products may not be paintable so check what you have first. It's unlikely to be Glasrock in a garage as it has a very smooth, almost marble-like finish, which doesn't really need to be decorated.
As you say it's a 'new house', I'm assuming you mean the garage is dry-lined with fireboards, rather than 'made out of fireboards', which are usually just taped and joined then left bare as a cost saving exercise.
EDIT:
Ah, this post makes me think I've got the situation wrong:
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local