GAC is also a carbon filter and it stops chlorine from getting to the membranes. But a GAC filter is only good for 2500 US gallons which is 9500 liters. As US gallons are smaller than Imperial gallons, that isn't much water, especially as you have to count the waste as well.
However, various companies supply different service life figures. The one I looked at says it has a service life of 5000 gallons, twice the figure on the label of the one Gardiner sells.
As these are used for all sorts of different applications, such as water coolers, it is little wonder then that they also specify that they should be replaced at least every 9 months.
Better to use a single Fiberdyne carbon filter from Gardiners as they will process about 38,000 liters before needing replacement. The filter can be used on its own as a sediment and carbon filter, but we also use a sediment filter first as our water is laden with sediment.