I glued a painting wood panel back to another small piece of wood to add hanging wire with gorilla glue. I now want to take these two wood pieces apart. What can I use?
A saw, a chisel, or brute force. Cured polyurethane glue is largely impervious to solvents, and if there are any exceptions to that, they are probably absurdly hazardous to people.
If you don't actually want the small piece, setting up a router jig to plane it off the painting would be a relatively tidy/controlled way of removing it.