Under the sales of goods act, the items has to last as long as would be reasonable for it to last (which as you say, is about 6 years), but the supplier is only responsible for it for the first year, or for items that failed straight away (which obviously you can't prove it did as you didn't try it).
For things that have failed beyond that period, it is then the manufacturers problem. So GG have no responsibility to replace it, it is down to GK to do that.