People have hit on most of them here, but here is another big one:

Fitness apps. Mainly calorie tracking, workout tracking and heart rate tracking

Health app

Sleep as Android

(No, gadget bridge is not a replacement for 99% of cases and doesn't even support the gold standard for heart rate tracking, polar H10)

For calorie tracking, the massive food databases required, barcode scanning, and crowd sourcing are generally not compatible with the open source community's privacy ideals. OpenNutriTracker has promise though!

For workout tracking, none of them have any device support and most of them are dead and abandoned. Not to mention heart rate zones, stats and training trends, etc... FitoTrack and Opentracks are good starts though.

And then a google fit alternative. Something that can integrate sleeping, workouts, heart rates, sensors, etc.. Data all in one aggregates place. It is a huge task and it makes sense that there is no open source alternative for it. Especially when the components aren't individually there to aggregate.