Is there an equivalent of gatherer for Duels of the Planeswalkers?
I don't think there is a searchable database like Gatherer that lists DotP cards, because most places that allow you to search for cards either get their info from Gatherer or MagicCards.info (or a thrid party that gets their info from one of those sources like MtGJSON) and neither of those sites allow you to search for DotP cards. There are however places that list all, or most of the cards from each expansion.

DotP 2015 + Expansion
DotP 2014
DotP 2013 + Expansion
DotP 2012
DotP (2009)