I've had my Gaz nickles for a good few years now and had them refurbed about a year ago. Recently they started knocking so I complained to Gaz suggesting that they should not be failing within a year of being re-built.
They fully accepted this without any quibbling and I duly sent mine off and they have returned me a new set of black shocks with the adjuster valve at the bottom. I've taken this to mean I have a free upgrade to my tired old shocks.
I know the subject of Gaz has been done to death on here - eg 'great customer service but why are there so many problems with them'. I also have to consider the fact that I'm lucky the VX is not my daily driver AND I happed to have a jack and a spare set of shocks to throw on whilst I'd sent mine off. There was also the hassle in having to box them up, take them to the post office and pay to send them back!
Would I but Gaz again? Probably not based on my overall experience, but if you do have them and they are failing I'd say persevere with Gaz and they will sort them for you.