On that note, if it is BOINC (not F@H GPU client), you can select the GPU WU and click on "Abort" on the left side of the software client.
On the other hand, if you have any GPU monitoring software (e.g. EVGA Precision), I'd suggest running it and see if the GPU is actually running the job (chances are it isn't after 12 hours). If aborting/killing the WU doesn't resolve the issue, you should check your GPU configurations.