Comment 28 for bug 1592177

When you want to file a bug against a certain package, you type the name of the source package, which is gtk+3.0 here. You can find the name of a source package from its Launchpad page at the top under the Ubuntu title (here for example https://launchpad.net/ubuntu/+source/gtk+3.0).

So you run:
ubuntu-bug gtk+3.0