First, unpack the wxGBTool archive
tar -xzvf wxgbtool-XXX.tar.gz
To compile wxGBTool, simply go
./configure --prefix=`myinstalldir'
make
make install
The installation directory can be the same one as where you installed Nero2d.
The configure script looks for a file called wx-config. If you installed wxWidgets in a non-standard place, be sure to add the path to wx-config (usually wxwidgetsinstalldir/bin) to the environment.
The installation procedure will yield a single executable binary, namely
bin/wxgbtool
This file can be launched in a graphical environment like KDE.