Beware of Fabsil on Goretex type membranes.
1, it can cause the taped seams to detach due to the solvent carrier.
2, it can knacker the breathability of the fabric and membrane, so whilst it keeps the rain out, it also keeps sweat in.
As has been said above, wash using a non detergent liquid soap, or soap flakes, this unclogs the membrane and restores breathability and prevents the outer fabric from wetting out, then use Nikwax wash in, it doesn't need heat to activate the crosslinked polymer.
If a garment has been previously washed in detergent it will have left a water loving, hydrophilic, residue that causes the garment to soak up water more easily and wet out so this has to be removed with a soap.
Check what the garment is made of, NikWax make different formulations for different fabrics, the Cotton Proof can be used as either a wash in, or can be diluted to 10% strength and directly sprayed on using a hand sprayer.