Code:
locate libgtop | grep pkgconfig
If you get something back from this command, add that path to $PKG_CONFIG_PATH
For example, if it comes back with /opt/gnome/lib/pkgconfig/libgtop-2.0.pc, you can put
PKG_CONFIG_PATH=${PKG_CONFIG_PATH}:/opt/gnome/lib/pkgconfig
in ~/.bashrc or ~/.bash_profile or ~/.profile...or maybe one of a few other files.