Can't use GTK Inspector for gnome-terminal
I've found the solution, I'm using GTK+ 3.22 for reference, just need to turn on this setting:
gsettings set org.gtk.Settings.Debug enable-inspector-keybinding true
And then the "Help" menu of gnome-terminal will show a "Inspector" item you just need to click it.
This is for the VTE widget only: Alternatively, if you are using jhbuild, the vte module has a test program for the vte widget (that gnome-terminal uses) so you can run it like this:
# inside jhbuild/checkout/vte
GTK_DEBUG=interactive src/testvte
Update:
On 20.04, the "Inspector" link is in the gnome-terminal window's "hamburger‑menu‑button > Advanced" submenu.