“Franch”? Is that a salad dressing? 
However much you want to believe this to be true, you’re doubly incorrect.
The Holy Blood and the Holy Grail says nothing whatsoever about the Catholic Church stealing land from the Merovingian kings. Such a claim would be too silly even for Baigent, Leigh and Lincoln.
Of course, it is true that the pope did seize control of the exarchate of Ravenna from the Lombards with the assistance of the Carolingians. But The Holy Blood and the Holy Grail doesn’t mention that either.
Crediting Baigent, Leigh and Lincoln with getting right information they don’t actually mention can hardly count as evidence for their reliability.