I track my workouts in another app — will Gretl see them?

On iPhone, yes, as long as that app writes to Apple Health. Gretl reads active energy from Apple Health and doesn't care which app put it there — Strava, Nike Run Club, Peloton and Garmin all write to it, usually behind a switch in their own settings called something like Connect to Apple Health, which you say yes to once.

To check what an app is actually allowed to do: open Apple's Health app, tap your profile picture in the top corner, then under Privacy tap Apps. Every app that has ever asked is listed there, with the data it reads and the data it writes. Gretl appears in that list too, with read access only — it never writes anything back.

On Android, Gretl doesn't read Health Connect yet, so active energy stays blank there for now.