The common use of "Dei Gratia" is as an ablative case, meaning "by God's grace" and was added to the royal title of kings to specify that they weren't subject to earthly jurisdiction. The nominative is identical to the ablative case so technically you could translate the phrase the way you did. Then again, in Latin in this sentence you would use the genitive case so it would turn into "Dei Gratiae". This is probably just me being too much of a Latin nerd, but the ones choosing the title for the OP seemed to be aware of the problem as well, as they translated the title as "Dei Gratia in the 5th Day". That doesn't make much more sense than the title I'm complaining about, but at least it avoids the problem of having to connect the "Compass" with "Dei Gratia" in a way that makes grammatic sense.