This is definitely a Gallic unit. There is a sreenshot with the exact same (or at least it looks exactly the same) unit fighting Romans in the Gallic wars action shots in the Roman preview. I have looked at the screenshots so many times that I recognized it instantly. I can't post any attachments for what every reason but here is a link to the preview.

https://forums.totalwar.org/vb/showthread.php?t=53849