You can get all gamepasses a user created using
https://catalog.roblox.com/v1/catalog/items?creatorTargetId={userId}&creatorType=User&limit=100
Then check ownership per gamepass with
https://apis.roblox.com/game-passes/v1/users/{userId}/owns-game-pass/{gamePassId}
Which should returns true/false even if their inventory is private.
Most of this stuff I’m pulling up looks a bit outdated, definitely old.
I’m out of my league with this one. Purely going off untested research.
Hope that gets you a bit further. Sure sounds like an interesting concept.