I don't think there is a great workaround for the diff settings issue. It shouldn't take us long to fix this. We will update this post when we have something.
If homebrew gcc-6 generates objects that are compatible with gdb, then you should be able to get full debugging support for it using SlickEdit. SlickEdit ships with a gdb for macOS (macos/gdb). Try using it as a command line debugger. If that works, you should be able to use SlickEdits GUI debugger with it. As long as homebrew gcc-6 command line works similar to standard gcc, then you can also take advantage of all of SlickEdit's build support. After creating a GNU C/C++ project in SlickEdit, configure the compiler here (Project>Project Properties, choose the Tools tab, select the "Build" tool, Click "Options...", set the Compiler field in the Compiler tab).