At the Garmin Connect IQ store https://apps.garmin.com/en-US/apps/00a77751-dbb3-4a88-a604-f652b3e4d90f#0
This app first displays your altitude above MSL. Then if you press select and confirm, it will set your current altitude as the ground height. The following display will give you your height above ground level (AGL) To reset your altitude back to above MSL, press the down button and then confirm.
Because this app requires the use of the down button, it currently only works on watches and devices that have a down button. If you would like me to rework the app to work with other devices, just drop me a line.
Feel free to jazz up this app. I created it at the request of a friend. I may be making small changes to it in the future
Cheers!