Not entirely sure sealing the interior surface of the plasterboard with waterproof finish is as good an idea as it sounds (though I may have misunderstood!)
I know most will know all this, but I've seen some dodgy practices (particularly by some French artisans) that suggest not everyone does. Some of those on our house and we're having to slowly undo the damage.
Outside walls of old houses were built with water permeable (lime) mortar and possibly with water permeable stone. Over the decades they have quietly sucked up moisture in damp times and dried out in the dry. The lime mortar is softer than stone so when the wall subtly changes size as it heats and cools the changes are accommodated by the mortar.
Inside the house, historically, they tended to have significant through flow of air and water permeable wall finishes so damp escaped from the walls, joined the general humidity created by breathing people, boiling kettles, cooking, drying clothes etc. and escaped with the flow of air through the building.
Now we come along and draught proof the property, trapping any humid air in, and then look for ways of sealing out the moisture.
Pointing and rendering externally with modern cements has risks. The cement can be harder than the stone so as the wall expands and contracts (very slightly) either the new mortar cracks, or the stone itself cracks. potentially damaging the integrity of the wall. This approach also traps any moisture rising from the ground inside the wall. That moisture either has to escape into the house or remain in the wall where freeze thaw action can cause further damage.
Best to let the outside wall breathe (lime mortar and/or render) and insulate and waterproof internally.
The insulation goes agains the wall. Then you put waterproof membrane. Water vapour penetrates from inside as well and will condense if it hits a cold barrier (if the waterproof membrane was between the wall and the insulation the moisture would condense in the insulation.) The plasterboard then goes on the membrane.
If you've done all that there is no need to seal the plasterboard against water penetration on the inside (unless in a specifically wet place like above a bath, sink, etc.) To do so risks trapping any moisture that does penetrate between the waterproof membrane and the outside of the board.
If you haven't done the insulation and waterproof membrane correctly and seal the interior surface instead the plasterboard will simply trap moisture entering from the wall. That will lead to mould inside the board and to it's eventual disintegration.
I have tried just painting emulsion on plasterboard, took 4 coats, and I hate painting. On the next job, using same paint, I put a 50% diluted layer up first, second coat finished the job! Lesson learned.
So, as many others have said, either whitewash, undercoat or (easiest of all) 50% diluted coat of paint to seal the surface.