HOW DO I USE GEMINI DEVICE WITHOUT DIRECTV STREAM

You don't. When you cancel the box becomes a boat anchor. Whether fair or not, I would like to see this changed. 

You can't. It will become a brick if you cancel. To use the device requires a subscription to load the service to access the Play Store and use apps. Most streaming devices load to a home screen where you run apps. The proprietary DirecTV device is preloaded with DirecTV Stream and requires you to log in to the app to access apps, etc. Since you will not be a subscriber, your credentials will be invalid.

I’ve posted this a dozen times over the years in response to this question (and ACEs and others posting the responses seen here):

From DirecTV’s Terms and Conditions for the device (https://www.directv.com/legal/directv-stream-terms-of-use-policy/)

• 3. Use of Device.Your access to and use of the Device requires You maintaining an active DIRECTV account. You may access the Device by entering the user ID and password associated with your DIRECTV STREAM Service. If you cancel your subscription to the DIRECTV STREAM Service, you may continue accessing the Device without an active DIRECTV account using the same or other valid DIRECTV user ID and password.
  

The phrasing there is confusing, but I take it to mean that you must have a valid DirecTV account to access the DirectTV service using the device, and must do that to get the device to start working, but if you’ve done that and subsequently cancel your account, it will continue to work for other apps. 

As far as I know no one has tried to see if that works and reported back, nor (if it doesn’t) contacted Support to find out why.  (I certainly haven’t, because I haven’t cancelled.)  

If it does work that way -  great.

If not…

The T&C is a legal document - it comes from DirecTV’s legal department (see the URL), and they can be held to it. It explicitly says “you may continue accessing the Device without an active DIRECTV account”, so if that isn’t true, people shouldn’t just throw up their hands and say “sorry, it’s a brick”, but contact Support to find out why and get it resolved.  If they don’t provide satisfaction (likely), go to Loyalty. And if that doesn’t do it, go to the BBB, which apparently gets responses from quite high up and will undoubtedly not be happy about possible (legal) repercussions.  Don’t just blow it off.