ᐅ Basement walls: Are gaps in the masonry normal?

Good evening.

Our basement walls have now been built, but only the horizontal joints have been mortared; otherwise, the bricks were just placed on top of each other, with some bricks having larger gaps in between (about 2cm (0.8 inches)). Is this normal?

I’m not quite following you at the moment. Do you mean the "gaps" between the bricks, essentially the vertical mortar joints? These are filled with mortar.

But how is the wall sealed further?

A double-layer bitumen coating with a fabric insert should be able to provide a proper seal in the end. 😉