For Mac users if Java is up to date, and the Gradebook is still not accessible, then perform the following steps:

1. System Preferences
2. Security & Privacy
3. General tab
4. Click the lock icon at the bottom-left
5. Enter credentials if necessary
6. Under "Allow applications downloaded from," select the bubble "Mac App Store and identified developers" or "Anywhere."
7. Reattempt to access Gradebook