The Gnat that sings his Summer's song Poison gets from Slander's tongue.
Imagine this: you're a gnat. You're just chilling out, making gnat noises, singing, hovering over a field with a bunch of gnat buddies. Then, somewhere on planet Earth, someone slanders someone else—they talk smack, false smack. If you knew about it, it would ruin your song—you'd buzz out-of-key, in an upset way.
That's basically the picture Blake's presenting us with. It's yet another "world in a grain of sand" moment.
Disrupting a tiny gnat's song reflects something bigger: slander, a lie told about someone (probably, someone who's trying to do good).
Note, yet again the modern trouble we'd have making "song" rhyme with "tongue." It worked for Blake, though.
The poison of the Snake & Newt Is the sweat of Envy's Foot.
You know the drill by this point: a little thing from nature, like snake or newt poison (there are poison newts out there), reflects a bigger human issue, like the emotion of envy. Yep—it's the "world in a grain of sand" repeated all over again.
Also, note that we're not talking about birds, mammals, or insects this time. We're dealing with reptiles and amphibians. So Blake seems pretty determined to use examples with all the animal kingdom.
Also, the sweat is from "Envy's Foot" (ew) for a simple reason: foot sweat is gross and so is envy.
Again, we have some old-time rhyme with "newt" and "foot."
Note: most of these couplets involve a metaphor of some kind. In this one, the metaphor is really directly stated because Blake says that the poison "is" the "sweat of Envy's Foot."