A whole lot of information about Josidiah Starym is provided in the Arcane Age "module" Fall of Myth Drannor by Steven E. Schend, published in 1998. This product has a detailed timeline of the fall, and Josidiah is featured many times as he eventually turns out to be the last Spell-Major, the head of Akh'Faer, arcane branch of Myth Drannor's army. Unfortunately the fate of Guenhwyvar is left in mystery, possibly meant to be fleshed out in a future product or to be imagined by individual DMs. According to this timeline (pages 17-18), in the year 674 DR, Josidiah returns from his 1.5-century-long quest for the long-lost elven blade Ary'Velahr'Kerym, Soldier's Blade. Guenhwyvar is no longer with him, and rumors are such that he traded the figurine during his quest.
Josidiah Starym returns to Myth Drannor to a hero's welcome as he brings not the expected Soldier's Blade but the Akh'Faer's Artblade, retrieved from a deep dragon's lair far beneath the Storm Horn Mountains. ... Josidiah refuses to discuss it, but many notice that he no longer walks with his magnificent cat Guenhwyvar, and some whisper he had to trade the cat's figurine of power for the blade.
I am afraid this might be all we know. In a Candlekeep post dated 24 January 2005, Steven Schend owns the rumor, arguably re-establishing its 'validity'.
I'm fairly certain I'm to blame for that rumor, as it's hinted at if not clearly stated in FOMD. I figured Josidiah wouldn't part with Gwen without an excrutiatingly good reason, and returning one of the fabled blades of power to Cormanthyr (stolen long before his day, if memory serves) would be one of the few. It's one of the hidden things in C:EoE and FoMD that I didn't want to underscore, leaving it as a reward for careful readers.