Gilgamesh is the genuine article, same VA and model and characterization as the one that appeared in Mobius where he has ENG VA lines about his encounters in FFV, VIII, XII and XIII-2. with the ones in IX and XI being separate characters (though Gilgamesh [FFV] has been to Vanadi'el at some point as he wields the Hauteclaire [FFXI] in his XIV boss model). As opposed to XIV Ultros being a separate and distinct entity for example.

Unless they make a distinction in any upcoming side content, they could have distinguished this Gilgamesh from the one from FFV onward like they did with Ultros [XIV]. That they decided not to means Gilgamesh is more than likely the one that Bartz encountered.