When Gold & Gems adorn the Plow To peaceful Arts shall Envy Bow.
Blake just finished arguing that war deforms the human race. On the other hand, peaceful arts reveal what's really most human about us. The plow is a good symbol of these arts because it's creative: it helps call up plants from the earth, which then go onto nourish people.
So, argues Blake, when people start viewing peaceful arts—like agriculture, or art and poetry (the things that Blake does)—as the main things humans do, then adorning them with gold and gems (i.e., funding them), envy itself will bow down and let peace reign.
Blake's a little like a PBS fundraiser here—he's an artist and he's arguing that art should have a more significant and better-funded place in society. You want to see more episodes of NOVA and Downton Abbey? Then deliver the cash.
A Riddle or the Cricket's Cry Is to Doubt a fit Reply.
This picks up on the same idea as the couplets back in lines 93-94 and 95-96: the pointlessness of doubting for the sake of doubt. It's not wrong to be critical or questioning, Blake's saying, but you need to actually be interested in finding out the answer, as opposed to just stating that there aren't any answers.
But before, Blake didn't really say how you should respond to doubters. He just said that trying to argue them into seeing the truth is a waste of time. Instead you should drop a riddle or a mysterious "cricket's cry"—kind of like when characters get hit with cricket noises after making a dumb joke in cartoons and sitcoms. Whip out a good haiku or a cryptic proverb—that'll show 'em. Maybe saying "A Riddle or a Cricket's Cry / Is to Doubt a fit Reply" is itself the sort of thing you should say in response. Blake might like that.