Well…the last time we saw Toshiro was in chapter 554, which was right after he defeats Cang Du.
And the only way Giselle can control a person is if her blood drips on them. The thing is, she said it’s easier to take control of Shinigami compared to Quincies, like Bambi. Toshiro could have died from his injuries fighting Cang Du, or at least passed out. Either way he was already in a weak enough state for someone like Giselle to take advantage of. Note that around the same time Toshiro passed out, Bambi started fighting Komamura, during that time, Bambi’s subordinates weren’t with here. This was probably when Giselle was turning people, since she would have had the time to do so. There’s also a possibility that Rangiku might have been turned as well.