The gPKPDSim app requires the SimBiology toolbox to run as it uses SimBiology models and associated functions for e.g. for simulation. The path you added does not refer to the actual SimBiology toolbox but just to the NCA capabilities. The NCA capabilities used to be an add-on for SimBiology prior to release 2017b, but have been incorporated in SimBiology starting with release 2017b. If you have the full SimBiology toolbox, it would live here (on Windows): C:\Program Files\MATLAB\R2018b\toolbox\simbio\simbio An easy way to see whether you have SimBiology is to go to the Apps Tab in MATLAB and see whether the SimBiology App is listed there (see attached screenshot)
or to type
in the command line. If you don't have SimBiology, we can set you up with a temporary trial license.
It seems like you already managed to install the GUI Layout Toolbox. If you haven't, the easiest way is to open the Add-on Explorer (far right button in screenshot) and search for GUI Layout Toolbox. When you click on the toolbox, there should be an install button that automatically adds this toolbox to your MATLAB installation.