I'm just posting this here so it's easy for folks to find.
The Dexcom G7 App is not compatible with GrapheneOS. This is just a fact and it's unlikely to change in the near future. If you want to read the history see: https://discuss.grapheneos.org/d/6751-dexcom-g7-connection-error
The Dexcom Clarity App DOES work on GrapheneOS. And it works well.
So, here are some choices.
1) Get a Dexcom G7 receiver. Then you can upload to Clarity using a computer.
2) Use the Nightscout xDrip+ App. xDrip+ connects directly to the G7 sensors just like Dexcom's App and it can be connected to the G7 sensors at the same time as a Dexcom G7 receiver, but xDrip+ doesn't upload to Clarity. I use Tidepool instead. It's all amazing, might require some knowledge to install, but you can search for all the answers. Download it at; https://xdrip.readthedocs.io/en/latest/install/download/
This is what I'm doing. I may dump the receiver and Clarity if my Endocrinologist can access Tidepool. So, you can just search Google for xDrip+ or Tidepool and do some research. I'll flag this post and try to keep an eye on it if there's questions.