Well, you have got a 'custom' gradient and you can get there in various ways, looks like you have ended up with an all black version.
Go to the tool options, click in the gradient icon to get the drop down menu and change to FG to BG (RGB) or even FG to Transparent.
Attachment:
gradient-type.jpg [ 31.93 KiB | Viewed 277 times ]
The real question is why does this happen. Sometime in the dim-and-distant past (maybe yesterday) you altered a default gradient and it becomes a custom gradient. By default Gimp remembers the last used settings even if the user forgets.
A quick animation of that:
https://i.imgur.com/qH2zPu4.mp4BTW. Are you really using Gimp 2.8 ?