Cousin relationships are defined by the common ancestor. Ones first cousin is someone with whom one shares a grandparent (in other words, your parents were siblings). So it sounds unlikely that you and someone's great grandfather could be first cousins. You certainly don't look that old in your profile pic, or if you are, could you tell us your secret:)

There are plenty of online tools to calculate relationships between two people with a common ancestor. A user-friendly one is http://www.searchforancestors.com/utility/cousincalculator.h....