Gray only won because Invel was an Ice Mage, Ice Resistance saved his ass from getting frozen like Natsu(though i think he has the physicals to break out) and his ass was saved yet again at the end of the fight when instead of getting flash frozen and shattered by Invel's Ice Raiment, he converted it and beat Invel with his own power. Not to mention having resistance to all of Invel's other basic attacks.
I still say Darkness Gray wins though based on his performance against END(and cause i think Invel is a low tier spriggan) but his victory over Invel was almost entirely reliant on the fact that Invel was an Ice Mage, so i understand why someone would rank him lower