Elyon
William could have used his talk with his Grandfather as a way to get Gavin more worshippers. He could talk about being reincarnated and say that he is a follower of Gavin the god of trade which allows him to get more powerful from not only killing monsters but doing other things. Even if he doesn't talk about reincarnation , he could tell the kids about Gavin and convert people. Gavin only has 1 other follower and his bonus seems pretty strong and extremely useful. William should be doing his part. Even if they can't sign a contract, Gods just have to be remembered. And maybe worshipping him will bring some affects to people.