The Lazarus svn does not contain the fpGUI sources.
You can get the fpGUI sources with

svn co https://fpgui.svn.sourceforge.net/svnroot/fpgui/trunk fpgui

Copy or link directories
<fpGUI dir>/src/gui ⇒ lazarus/lcl/interfaces/fpgui/gui
<fpGUI dir>/src/corelib ⇒ lazarus/lcl/interfaces/fpgui/corelib

For more details see:
http://wiki.lazarus.freepascal.org/fpGUI_Interface

