Gaea's Blessing is good as a mill hate piece, it's just a bad card overall which ironically proves the point that Commander player will do anything but sleeve up counterplay if it means sacrificing gas.
Generally a mill deck will try to win before you have drawn 50 cards, which means that Gaea's Blessing (or a shuffle titan) is more likely to be milled than drawn. If you really wanted to, handfixing cards like Scroll Rack can also be used to at any time deliberately put Blessing back on top in response to mill if you happened to draw it.