I didn't really think of that, but I suppose the Eco looks a bit like the GAC. The Eco doesn't perform like GAC, though. Eco Complete comes saturated with the nutrients that your plants need, like iron, potassium, magnesium, and others. I found that my KH value increased slightly when I first set up my tank with Eco. I appreciated the buffering effect since my tap KH was rather low, and I was injecting CO2.
GAC won't provide nutrients to the plants or buffer the water column like Eco. Once carbon is used, I would throw it away rather than recycle it in the aquarium. It could leach out the impurities that you used it to remove.
I have read that sometimes Eco Complete, after a year or more, may leach nutrients back into the water column, but I don't have any way to prove that. My tanks have all been set up with Eco Complete for over a year now and they are all doing great.