Hello,
After a quick cursory search, I found only dated information on this subject, so apologies in advance for a potentially redundant post.  

For context, I'm using an M600 + Ronin MX w/ aftermarket camera for photogrammetry work.  

Does anyone know if it is possible to retrieve flight telemetry (specifically interested in battery levels compared to GPS positions) from a GS Pro mapping flight?  Using the .DAT file viewer does not really show what I'm looking for, as battery inputs and GPS lat / lon from GS Pro don't seem to be logged.

If this is possible, what workflow would be best to get that info?  

Hi there,

as we speak, this is not possible with GS Pro as there are no retrievable flight logs form that app.
Requests have been made to implement this to DJI for more than a year ago in this forum, but were never heard.

So, to accomplish your task, i suggest to use other software such as Autopilot. They are creating logs which can processed later (I.e. Airdata or whatever to your liking)