If you have any access to the sources, you might just release. Any derivative work of a GPL-licensed is already GPL. Also, as somebody else said, let the origi

If you have any access to the sources, you might just release. Any derivative work of a GPL-licensed is already GPL. Also, as somebody else said, let the original authors know. Funnily related: years ago I downloaded wordpress/joomla themes off bittorrent to study them. I was surprised to see that most/all of them were GPL-licensed, since they all were more-or-less based on some other GPL-licensed theme. I could have just used them in prod no problem, worst case scenario I could remind them that the code is GPL-licensed.