If one considers gay marriage a noteworthy success, how much credit does Obama really deserve? He appointed Sotomeyor, but any Democrat would've appointed a 5th vote justice. It's like giving Ike credit for the Brown school desegregation decision.
Obama had a big impact on public acceptance of gay marriage. Sure, approval was already moving in its favor, but he was instrumental in ending DADT, and his presidential victory after his endorsement of gay marriage opened the floodgates for elected officials to support it. I also suspect that the entire tone of the court case would have been different if an administration had decided to keep continue defending DOMA. Who knows if Obergefell vs Hodges would have even reached the Supreme Court?
It's not as sexy as signing legislation, but Obama had a huge impact on the cultural shift in the nation that allowed gay marriage to be legal nationwide.