Welcome to the forum! 
The application you are looking for is available from the AUR. As such, installing it will comprise the local compilation on your machine of the source code. Depending on your CPU, your amount of RAM and the size of the application, this may take some time, but at least it’s there.
In order to install it, you must first fully update your system if you haven’t done so already, because there was a major update earlier today. 
Close as many applications as possible, open up a terminal window, and issue the following command…
sudo pacman -Syu
Let the update finish ─ don’t power off or reboot your computer while the system is updating, because you won’t be able to boot anymore. Then, when the update has finished, issue the following command in order to install AdaCore…
pamac build gnat-gps