Sometimes, while downloading an app, downloading will freeze (i.e. gray icon, not downloading or installing). To fix this, respring your iPad. Press and hold down the lock button until a “Slide to Power Off” slider appears. Move the slider to turn your iPad off. Now turn it back on by pressing and holding down the lock button again.
When your iPad is back on, all frozen downloads should have either canceled or installed (Note: App locations may also change. This is because the resprining process causes youriPad to think that all apps, whether they are new or being updated, are newly downloaded).
Have fun with your iPad!