Jandra would actually be the one to die from StW here, but you could have easily reversed the order and nothing would have changed.
The question I have, though, is whether Jandra dying causes the gargoyle to “die”. It should be “destroyed” (as it is not a demon), and I believe that means its indestructable-ness would prevent that and it must be exhausted (so it couldn’t attack).
The case for Vandy doom triggering the death feels very similar to the Jandra case Rules Questions thread - #408 by Pooh_Bear, and thus I think your attack with the Gargoyle is illegal. Thoughts?