Why Whatsapp images and videos are not showing in my gallery?
I believe the trick here is to somehow get the media scan to run again. I would normally expect a reboot to take care of this, but perhaps a more explicit instruction will work.

As per this answer, I'd suggest executing the following ADB command:

am broadcast -a android.intent.action.MEDIA_MOUNTED -d file:///mnt/sdcard


Also, be sure to check that .nomedia doesn't occur in any of the folders (or subfolders) of Whatsapp's media directories.