I have already attempted to answer this question in several places. I have explained in my History of the Franks how the Franks of Wallonia adopted Latin, which they corrupted into becoming French. In Merovingian times, the Frankish court was based on Tournai, at the northwestern extremity of Wallonia. The Carolingian dynasty emerged from the region of Liège, at the northeastern extremity of Wallonia. These two poles of royal power formed the northernmost boundary where Latin/French was/is spoken by a predominantly Germanic population.
In my Belgian toponymy I have analysed most place names in the country and sorted Germanic from Latin and Celtic names, and created maps to visualise the location of Germanic settlements in Wallonia. It it also quite obvious that the regions of Liège and Tournai-Mons are far more Germanic than central and southern Wallonia, in accordance with the Frankish centres of power.
Finally, I have made the breakdown of Y-DNA frequencies for Belgian provinces. However the sample sizes are far too small in Wallonia to mean anything at present. Overall Walloons appear a bit less Germanic than the Flemings, but nevertheless a bit more Germanic than Gallo-Roman. This seems to confirm my hypothesis that there was at first a Germanisation of Wallonia, followed by a Latinisation of the Franks in Wallonia. The alternative is that the Franks of Wallonia adopted Latin/French because they were overwhelmed by a native Romance majority. Genetics clearly doesn't support that Walloons are overwhelmingly of Gallo-Roman descent. That much is obvious by mere anthropological observation. Physically Walloons are much closer the the Flemish than to the "average" French (which of course does not include people from French Flanders, who are genetically Flemish).
* Language and culture typically go hand in hand. It can happen that an ethnic/genetic group shift language and culture, but there aren't many cases of people adopting a language without the culture that goes with it or vice versa. And Belgic people were undeniably a single cultural entity of Celtic rather than Germanic affinity.