You do not need to use xcode (but you do need to install it). If you have some programming experience (or are comfortable on the command line), you might be able to get it going pretty easily.
Then you'll need to build libindi to provide the libraries we need to link against:
mkdir -p ~/indi_projects
cd ~/indi_projects
git clone https://github.com/indilib/indi.git
cd indi
mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_BUILD_TYPE=RelWithDebInfo ../
make
sudo make install
Now that libindi is installed, we can build indi_gamepad and install it.
cd ~/indi_projects
git clone https://github.com/rickbassham/indi_gamepad.git
cd indi_gamepad
mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_BUILD_TYPE=RelWithDebInfo ../
make
sudo make install
Then you should be able to restart KStars and see the gamepad (hopefully).