I think this is purely a case of semantics. i.e. different names for the same thing. gram formula mass was a term still used when I was at school and merely idicates that the atomic or molecular weight (mass) is in grams. The terms atomic/molecular weight (mass) began to be used at the same time and were assumed to be in grams rather than stating that. I suspect that these days that atomic/molecular mass unit is the Dalton and therefore are relative to the current atomic mass standard [it used to be 1/12 of the mass of 1mole of C12]. For all everyday practical purposes they are the same.