I will confirm that others have noticed the liquid in the Eco Complete bag raised their Kh, by about 3-4 degrees (they didn't report changes in Gh).
This is because the water is meant to help buffer. Doing a few water changes should bring the levels back to 'normal'. Again, it's only the liquid 'amazon water'...not the eco complete gravel that's causing your change in parameters.
It seems this isn't consistent though. I didn't see a Kh increase in the tank I used eco in, and others agree...yet several reports are just like yours.