Question: could not use the gradient tool because the scratch disks are full?
Answer: If Photoshop 2019, or earlier, cannot launch because the scratch disk is full, hold down the Cmd + Option keys (macOS) or Ctrl + Alt keys (Windows) during launch to set a new scratch disk. You can tweak scratch disk settings in the Preferences > Scratch Disks section. ... To apply the changes, restart Photoshop.