When you use the Restore menu, and the “Create a New Database to Restore to” window opens, you need to use a Label and DB Name that do not already exist on your system. You cannot restore a backed up database to an existing db name.
What version of Resolve are you using? What version of OSX? What CUDA version? Are you sure your Energy Saver settings are set to not use Automatic Graphics Switching, or if you have this option, that Graphics is set to Higher Performance?
Using Resolve 10.0.2, set the Resolve Preferences – Video I/O and GPU – GPU processing mode: to OpenCL, save the prefs, restart Resolve, and see if that has any effect.