You don't need to install anything extra to compile LCL for Gtk 2. Everything necessary comes with Lazarus.
Just open Lazarus, and go to the Menu: Tools --> Configure Build Lazarus
Set all targets to NONE and only LCL to Clean+Build.
Select Gtk 2
And Build Lazarus.
Now, open a project you want to compile for Gtk 2. Go to the Compiler Options menu option. And select Gtk 2 as Widgetset.