Really, GBRs are said to require very specific water requirements, which, in nature, is simply impossible - water temperatures constantly change as does pH through any number of factors - and as such means that the fish will probably do fine, if not thrive, in a larger range of conditions than are commonly accepted, baring of course poorly breed and thus weaker fish, which will be more likely to become sick in less than ideal conditions. Bolivian Rams are known to tolerate a wider range of conditions but GBRs will probably be fine as well. Give it a shot and keep us in the loop about how it goes. We'd all love pictures once they are in the tank...