If you're thinking it was for payback, don't, I've only been eligible two times since the switch to the current model however long ago that was and I think it's absurd that doing a one goodybag network test makes me eligible for any payout.
(That's part of the reason I campaign against payback and regularly advocate for it's abolition).
I would be lying however if I denied that the fact that giffgaff are literally throwing money at me hasn't played a part in my decision to use them as my backup provision because as long as they can do the business I'd feel too guilty spending it elsewhere.
Honestly, I'd have actually preferred to take the EE Black Friday SIM only deal, (£30/60GB), or the Virgin one, (£20/200GB), as my backup, or even the Three one despite their slow speeds, (£20/unlimited), and I almost did but giffgaff throwing money at me kinda makes me feel obligated to use their services again.
I will be making sure that I'm not eligible for payback next time around so I don't feel that obligation. Maybe I'll still choose giffgaff as my backup at that time, I don't know, but it'll be because they merit the position if I do rather than because I feel obligated to do so, (I know, I know, but my conscience is a weird beast).