Gildarts said the real God Serena was powerful and he would've been eager to see who'd win between the two of them, so I think regardless of who wins, they can be considered CLOSE.
If we look at their stats, I know we haven't seen much of God Serena, but seeing how he got 1-shotted by Acnologia, I'd say he's less durable than Gildarts (who at least survived such an encounter). I KNOW Acno didn't take Gildarts seriously since he had no-link with dragons, but when he attacked Tenrou Island, Gildarts said he wasn't using a thousandth of the power he saw him do, implying that while Acno didn't take him as seriously as a dragon, he did more than just toying with him (don't ask why he'd treat Gildarts differently, Gildarts confirmed he did). Plus I consider Alvalez Gildarts much stronger than Pre-Tenrou Gildarts (before he was wounded), while Acno can't have made much progress in 8 years since he's over 300 years old (thus anything he did to train, he probably did during his centuries of existence as well, I don't see what 8 more years would change).
Also, we haven't seen ALL of God Serena's dragon slaying magics, so I wouldn't be surprised if at least one of them focused on defense, but Gildarts also has a defensive Crush "barrier", plus his Crush magic can counter (or at least weaken) a lot of spells since it can destroy both physical matter and magical energy.
So all in all, Gildarts seems to have better defenses.
In terms of offense however, God Serena obviously has more range and AoE, more versatility since he has 8 different elements, and easily overpowering the 4 Gods of Ishtar proves he isn't a weakling. His 8 dragons attack should be much more powerful if it was the REAL him who cast it (not his Historia), and he probably has a DF mode (which likely triples his power like it does with Natsu). Gildarts may have highly destructive magic, but I don't think he can reach that level.
MY opinion is stalemate or Serena extreme-diff, but knowing Mashima, if this fight really did happen, he would've made Gildarts win extreme diff like with Laxus VS Jura.