I would assume so since GH is made up of either calcium, magnesium, or both. Someone on APC had a Mg deficiency with a high GH. His tap water was almost all Ca and no Mg. Dayton's Comprehensive Water Quality Report lists 28ppm of Ca and 19ppm of Mg.
If you would like, bring over a sample of your tank water and we can run a Ca test on it. From there we can figure out the Mg content. When Tom was here, he did mention that I could probably use a little more Mg in my tanks in the basement!