Why would Jean V have gone to such trouble to frame him? Gilles was broke and had already sold the Duke the two border castles he particularly coveted.
Gilles had what we would nowadays call cash-flow problems, but he was far from bankrupt. In fact, in return for Champtocé and Ingrandes, Jean V returned the entire Barony of Rais to him (incidentally indicating how valuable those particular castles were to the Duchy). However, a clause in the contract meant that Gilles could buy them back at any time within the next six years for the price he sold them for. Modern historians dismiss this because Gilles could never have afforded to do that; however, we know that the Duke had spies in his household, and would have been informed that, in late 1439, Prelati claimed to be close to finding the Philosopher's Stone. If Gilles could turn base metals to gold, he could certainly buy back those estates.
There is also the consideration that Jean V was breaking the law; the Duke of Brittany was not permitted to enter into property deals with his vassals. For this reason he often used straw men as a cover for these illicit deals; Geoffroy Le Ferron, for instance, was acting as his proxy when he bought St-Étienne-de-Mer-Morte. Jean V's claim on the estates he had bought was precarious. In fact, a protracted series of court cases after Gilles, the Duke and the Bishop were dead saw all the disputed estates restored to the Rais family.
Further reading -
If there was a plot, why was it so elaborate? Why such extravagant accusations?
The main focus of the plot was to obtain Gilles' properties legally. However, political forces were also at play. Jehanne had already been burned – not for witchcraft, as is often supposed, but for heresy. Gilles had been the pre-eminent French captain in her army, he was closely linked to her and apparently tried to save her from her captors. He had also staged, and possibly had a hand in writing, a Mystery play in which this condemned heretic was depicted as a messenger from God. This in itself verged on heresy. He was put on trial in Brittany, an independent Duchy which more often allied with England than France. The Duke had close family ties to the English throne, his mother, Jeanne de Navarre, having remarried Henry IV of England; the Bishop was a lifelong and passionate Anglophile. To smear Gilles was to smear Jehanne, and to assert that Charles VII owed his throne to a pair of heretics and black magicians, one of whom was a perverted murderer. The mud did not stick to the maidenly Jehanne for long but, as we know, the smear campaign against Gilles was infinitely more efficient.