Gripfill will only be as strong as the substrate – so if you stick it to a painted surface, it will just pull off with the paint. Plastered surfaces are better, but beware a skim coat on plasterboard as that could pull off too.
Do you know what sort of tanking system was used? External or internal to the basement wall? If external, then you should be able to plug and screw directly to the wall, but don’t get this wrong as you’ll spring a leak!
Gripfill for teh worktop could be trickier. Depends how many sides you are supporting. Best bet would be to form a ‘table’ structure with four batten legs if possible, so the gripfill is only holding the worktop against the wall rather than supporting any weight.
*edit – make sure you prime the skirting both sides. Basements always get damp, as much from lower ventilation as leaks. If your skirting gets damp it will expand, bow and pop off the wall. Priminmg all round will reduce the likelihood of this (particularly cut edges). Also use softwood rather than mdf if you can as mdf is more prone to water expansion.