>>101539310
>how does the automaton make herself cum
With her hands? Toys? Gigi's unconscious (sleeping) body?
Though, judging from your question, it seems that you're assuming she doesn't have anything down there, which brings us to her other input modules: nose and mouth.
She could offer herself to be Justice's sole maid/houseworker just to get her hands on Gigi's unwashed laundries.
Maybe she always kept a pair of Gigi's stanky spats in her nightstand, sniffing and licking on it before sleep or during her personal time.
There's also the possibility of Ceci tiptoeing over to Gigi's room every time she's in the mood, saying nothing before pushing themselves into the bed, making out with a lot of fondling and french kisses.
She may not have a womb, but she can still reach an orgasm (how is that even possible, I do not know)