It would appear that gravity drag has a exact formulation for an instantaneous moment in a rocket launch.
I'm tempted to think that some calculus could get you an exact number for Delta-v of an entire launch to orbit knowing only the starting and ending values. For any given launch, we have know things like:
- altitude at the launch site
- altitude of the orbit (assuming circular)
- the time and position when it reached full orbit
It seems non-obvious whether this information is sufficient to get an expression for the gravity drag or not. Given the same initial/final location and timing, do all approaches to orbits have the same gravity drag?