Immature GBB 1 - Boy or Girl

You won't see a flap when ventral sexing (the flap is located on the interior of the exuvia between the first pair of book lungs), what you're looking for is either a "hairy dot" (gonopore) for males or "lips" (gonoslit) for females.
That said, I want to guess female, there are no obvious "lips" but the first pair of book lungs are far apart and angled, this is a tricky species to sex this way so don't take my word for it, you're better off waiting for a moult to confirm 100%.

It's difficult to tell with this picture. Maybe if you could get a picture of it on bare glass because that web makes it difficult to see.