RE: How can I remove an entity in order to remove Dynamics 365 Gamification?
So basically, it is clear from the image where your culprit is.
1) Go to make.powerapps.com and select Apps. Find that app (Dynamics 365-Mobile Custom) and from the 3-dot menu, edit that app (assuming that you are an admin).
2) When the app editor opens, first open the site map in that editor and look for the Player entity somewhere there, find it, select it and delete it. Save and publish your sitemap and close the sitemap editor.
3) Now you are back into the app editor, from what you see there, if you find a reference to an entity from that gamification solution, remove it and save and publish your app. When done, publish all customization and try to delete the solution again.