I heard the same and Maria Antonia of Austria was pratically the best match in the world at that time...Actually, Cosimo probably would. The reason he agreed to Giovan' Gastone's OTL marriage (if I remember Acton's book right) was because he wanted a cadet branch of the Medici to expand Medicean influence OUTSIDE Italy.
and Gian would be coached to be eternally grateful for a such wondeful match and they would likely get along well enough...
With power, her birthrights safe, already in possesion of her heritage and with a grateful husband interested in presiding a brilliant court and promoting arts I think Maria Antonia would be much more happier, pleasant and tolerable than OTL..
Gian Gastone would be King Consort to one of the greatest kingdom in the world (and with an higher rank than his father and brother) and sure the princess who he has married is a little older than him and maybe too much proud of herself and her blood but she has good reasons for it and he know he is a lucky man to have married her plus her reign is big and full of beautiful cities so he is at least satisfied with his life
Maria Antonia also was intelligent and well learned, loved music and was introvert and serious and OTL suffered and was offended by her husband constant infidelity...
Gian Gastone was sensible and intelligent, was well learned, interested in art, antiquariate and culture and most of his melancholy and (and the morst of his debauchery) will be butterflied with a an early match of this importance (his OTL debauchery started likely as consequence of his very unhappy wedding).