Actually I thought showing the mask was excessive. When the fight started I already guessed it could very much beGaius judging by the fight stance and the signature abilities the legatus have. When the cutscene zoom in on the gunblade and the subsequent acknowledgement by Gaius to have known Alphinaud before cemented my guess that he was indeed Gaius van Baelsar.
I must admit, I haven't felt giddy like a little kid for a long time, that reveal scene got me all excited for this game all over again.