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

cd lcl/interfaces/fpgui
svn co https://fpgui.svn.sourceforge.net/svnroot/fpgui/trunk/gfx gfx
svn co https://fpgui.svn.sourceforge.net/svnroot/fpgui/trunk/gui gui

