But by following this logic, Gardik should've outlived four Redanian Kings (who let's say all died approximately around their fourties). That should make him way over one hundred at the time of Falka's rebelion. Goidemar's age doesn't really matter (he should be around the same age as Vestibor or even his son Radovid II) as he's the one who married Riannon (and he didn't marry her at a ripe age of 70, as he was a pretty young prince when she chose him).
Mathematically nothing adds up. There should be at least one or two generations betwenn Gardik and Geddes, but there aren't. The only thing that've make sense is that Gardik took potions to prolongue his life, which again is nearly impossible as mages kept that secret to themselves.
Although what you point out is reasonable and surely happened historically, in this case it is outright impossible if we exclude magic.
I'd like to hear further from you, cheers!
PS: There's also the fact that Vridank was in his mid twenties or early thirties when he took Riannon in (that's another twenty years to add to the equasion), as his brother was already fifty when he took the throne of Redania after the whole massacre.