Grouting loosely laid terrace slabs "waterproof"

Just think about how much water comes through the joints. When you finish thinking about it, you'll realize that your question is nonsensical ;) .

Assistance:
The joints make up about 1% of the area (roughly estimated with 60x60 cm tiles), just to have a calculation size. Additionally, there is a slope installed.
What do you think will actually come through the joints during a two-day continuous rain? More than 35 liters in 6 hours of rain per square meter is a storm. The majority runs off directly. So you will actually get a water ingress through the joints in the milliliter range. Even after two days of continuous storm rain, nothing comes through (at least no relevant amount).

Just think about how much water gets through the joints. When you’re done thinking, you’ll realize that your question is nonsensical ;).

Help:
The joints make up about 1% of the surface area (roughly estimated with 60x60 cm tiles), just to have a calculation basis. There is also a slope installed.
What do you think will actually get through the joints during a two-day continuous rain? More than 35 liters in 6 hours per sqm is a severe weather event. Most of it runs off directly. So you will actually get water entry through the joints only in the milliliter range. Even after two days of continuous heavy rain, nothing gets through (at least no relevant amount).

That is of course not true. Where does the water run off the slabs? Exactly, into the joints.

The joints are about 1mm wide.

Although the slabs as well as the gravel + crushed stone have a slope away from the house, this does not apply to the poorly water-permeable soil beneath the gravel (gray clay/loam) ground.