To be honest I'm not entirely sure what Zeo is (apart from a brand name). The last time I checked they don't put their ingredients on the bottles so I won't touch it. GAC stands for Granular Activated Carbon. As far as Ca Reactors go this article should help you decide which calcium carbonate supplementing system would work best for you and your needs:
http://www.advancedaquarist.com/2003/2/chemistry
The options discussed are:
Limewater (aka kalkwasser), used in a reactor or not, and with vinegar or not
Calcium carbonate/carbon dioxide reactors (CaCO3/CO2 reactors)
Calcium carbonate used without a reactor
Calcium acetate
One part inorganic salt mixtures
Two-part liquid additive systems
Water changes