So if I understand correctly the glucose monitor uploads data to a Nightscout server and Intervals.icu could get data for the time range of a completed activity and attach it to the activity? Or you could upload a GCM file and do the same? Is there a standard format for such files?
I do have attaching arbitrary time series data to an activity via CSV upload on the todo list. Another way to achieve this would be to make it possible for activity stream scripts to call out to other services (e.g. Nightscout).
I see there is also a Garmin Connect IQ app for Dexcom: https://www.garmin.com/en-US/newsroom/press-release/wearables-health/people-with-diabetes-can-now-view-dexcom-cgm-data-on-their-garmin-smartwatch-or-cycling-computer/
Presumably that writes data to the fit file which would make it very easy to view in Intervals.icu.