GotCGM does not currently support Dexcom’s Over-the-Counter (OTC) CGM, the Stelo. There exist third-party (unaffiliated with GotCGM) solutions that allow patients using a Stelo to upload the Stelo CGM data to a Nightscout-compatible backend. These are not FDA-reviewed nor approved solutions. You can search
GotCGM does expose a Nightscout-compatible API from Settings > CGM Settings, then tapping Other CGM. That API endpoint can be provided to any app that knows how to write CGM values into Nightscout to upload the CGM values it collects into GotCGM.
If the above approach sounds like it might work for you, you can search the web to find more guidance. You may want to add “iPhone” or “Android” to your query depending on your phone.