Slip revealing? You perv.
I was waiting for a post like that from you xD
I think Gildartz implied that GS and him were on the same tier / same level. But he didn't know about the result of a 1v1, we can only speculate. I think these claims by Gildartz were made (i mean, created by Hiro) to make us understand that even if beating a GS Historia might be easy for Gildartz, he would still be able to fight toe-to-toe with GS and the odds would be 50/50.
In terms of feats, GS didn't show good reaction speed or speed in general. No durability either (i consider him vastly superior to GoI so him tanking their attacks is nothing special). I tend to agree with you, i think Gildartz would have won against real GS.
However i think you're having Gildartz too high in esteem (as usual), as it would definitely not be easy for him to defeat GS. It would be a great and close fight. Not a stomp, for either side.