Suzaku destroys him. The reason Gildarts was said to be the strongest member of FT in the beginning of the quest could just be because Natsu couldn't activate Dragon Force at his will at that point in time. I am more inclined to believe Natsu learnt to use his Dragon Force at his whim after he ate Ignia's flames , i.e. combining Ignia's flames helped him gain control over his power better. If you look at it Natsu had been placed in life threatening circumstances a few times before , such as when Zeref trapped him in his magic and left him completely vulnerable to a lethal attack , or when he got swallowed by Mercphobia's ocean , DF didn't activate back then. That combined with Natsu's emphasis on Ignia's flames points to the conclusion that he gained authority over his powers just recently.
That said , I am only considering Gildarts' Alvarez performance as of now since that's all we've got to go off at this moment. But I do believe Gildarts can give a good fight or even defeat a Black Dragon Slayer Knight when he comes back. As far as the issue of defeating the BDSK's is concerned , from my perspective it will just end with us not seeing their Dragon forms ever. They may be Acnologia tier dragons when they tap into their full potential in their Dragon form , but we do know transforming into a human significantly limits them from drawing out their entire ability. The same holds true for Acnologia as well. And we also know that being defeated in the human form also results in them being equally injured regardless of their Dragon form , as evidenced from Selene's defeat. We will most likely get the FT top tier class reaching Human Acnologia level by the time they defeat the BDSK's , and the knights will taste defeat in their human forms without getting the opportunity to tap into their dragon forms at all , pretty similar to what happened with Selene. Could be plot due to which they don't use their dragon form , but hey we're pretty used to it at this point , we know what to expect.
All in all I wrote this to show that the idea of Gildarts beating a BDSK isn't completely out of the world. He however lacks the feats for that at this point in time , and speculation should be avoided for the sake of the tournament. Gildarts doesn't need to be Dragon Acno level to beat them in their human form. But as of now , he gets stomped.
