Duel licensing the contribution is always an option. you can add contribution agreement with statement like "all the contributed code will be under duel licens

Duel licensing the contribution is always an option. you can add contribution agreement with statement like "all the contributed code will be under duel licensed, original commit will should grant all rights and ownership of it to the XYZ INC/LLP. the copy of it will automatically be commuted to GPL licensed source repository under original contributor's name. "