In my opinion Jacksharp has it about right.....
The two products he mentions are recommended for Gortex BREATHABLE Garments plus
'Sympatex' 'Permatex' 'eVENT' and Nikewax 'Anology' - this from the Nikewax instructions.
I do a lot. and I mean a lot of walking in ALL weathers and Gortex garments are without doubt what protects me from the elements. However, from time to time they do lose the 'waterproofness' they are famous for and once a year they NEED 'reproofing' which involves the use of the above products.
Only this last Tuesday I came back from a long walk in very heavy rain and found my walking jacket was damp on the inside so it will need to be reproofed. Once done (and this involves the use of a washing machine for a couple of hours!) they are as good as new and from my considerable experience a jacket cared for in this way will last many, many years.
For the record my preferred choice of manufacturer is Bergaus. Expensive yes, but IMHO worth every penny as they are well made with ample pockets, built-in hood etc.
All the above refers, as I say, to BREATHABLE garments which is not to be confused to non-breathable! For these, as Peter and others have said, a quick hose down with a hosepipe and a soft brush does the job. HOWEVER, with non-breathable stuff you have to face the prospect of sweating within the garment which, if you are moving around from swim to swim (and you carry a few extra pounds!) can be excessive and very uncomfortable particularly in winter when sweat soon turns to very cold!
I don't use the 'non' stuff for this reason, and whilst it may be cheaper initially, in the long term it's disadvantages are many. However, (again) they do have a use if you just need a quick something to keep off a shower and carry without much weight,. Those made by 'Pakamac' for example provide excellent temporary cover BUT they sweat something terrible in almost any weather making what you're wearing underneath 'wet' by any standard.
When walking (in summer) I carry such an animal simply because it rolls up to almost nothing and weighs ziltch BUT they are only for the 'caught out' situation.......IMHO
As you can see I have done my own checks before deciding what best suits for a given situation and I strongly suggest you do the same, ignoring, if the pocket allows, what the product costs but more what you want it to do FOR YOU!
Look long term is my advice but let me just say I have many breathable jackets that go back years, still functional simply because they just don't wear out!!
ps .....and for gawd sake READ the instructions before you buy and more so before you 'reproof' if only to be sure it can be done - okay??