Yagn's makes Maggie a Gadget, so she is a legal target.
She would unboot (and stay unbooted), the same way a dude aced by a Shotgun would stay aced.
She would lose the effect allowing her to use her ability again at the end of the shootout, as the effect is a shootout play and therefore only last until the end of the shootout.
Also, if it's during Maggie's job, she'd go home booted if it succeeded- so it's a regular shootout only scenario.