Unfortunately Verditius, we know little about the laws and customs in Gaul. Even before Caesar, Gaul was heavily influenced by the classical cultures she'd come in contact with and was already moving to a more advanced culture. It would be interesting to theorize on what sort of culture Rome would have found if they'd waited 100 years before moving into Gaul.

What we do know is that comparisons between the two Celtic law systems that we do know of, Brehon and Hwyel Dda, show us a great deal of similarities. So, it wouldn't be that far of a stretch to say that something along the lines of fosterage program among the insular celts could have been found also on the continent.