ᐅ Painting a precast concrete garage white / light gray / gray?

That may be the case. They offer two concrete finishes, plaster, and possibly other surface options separately priced. This is not a problem and is actually more transparent than some all-inclusive quotes with just one total figure. I think this is perfectly fine; in the end, an individual selection is necessary (size, materials, roof, drainage, surfaces, interior fittings, door, motor, side doors and windows, etc.) which you then compare as an overall price. The interesting question now is whether you can choose the concrete finish without extra charge and paint it yourself to save the costs of plaster, without damaging the exterior shell of the garage, for example, from rain, etc.

What does the garage manufacturer say about their standard surface finish?

Is it ready as is, or does it require additional on-site treatment?

What is the quality of the concrete surface like? It should be reasonably good if you only plan to paint it.

From the scope of work description (this is about Concept Beton):

Concrete surfaces
Interior: surface hand-troweled; exterior: surface formwork-smooth. The surfaces comply with fair-faced concrete classes SB1 and SB2. Alternatively, coating with roller-applied plaster, standard white, or for on-site cladding, plaster or paint is possible.