The GUI requires two QT5 packages. Install them by running the following.
sudo apt-get install qt5-default libqt5webkit5-dev
Compile the GUI by running the make in legup/gui/LegUp_GUI, or in the base LegUp directory. Once compiled, add the GUI to the path, and also add the schedule viewer to the path if you haven't done so before.
export PATH=~/legup/gui/LegUp_GUI:$PATH export PATH=~/legup/gui/scheduleviewer:$PATH
Using the GUI also requires that you have the following environment variable set.