A friend of mine uses a gesture-based app on Linux and I'd like to port it to OS X. The code is open-sourced and available on Github but I have no idea where to start.
Got some tips, articles that could point me in the right direction?
First, Apple still has a X11 server on MacOSX, so you can just compile and run your X application there.
Otherwise, learn Objective-C and Cocoa, and rewrite the user interface. Works better when the application is well modularised to begin with.
You could start with macports or homebrew. Those tools allows you to install open source software in OS X.
One example is XQuartz the X11 port for OS X http://xquartz.macosforge.org/trac/wiki/Releases
If you know what are you doing probably this is a good point to start, and if you need other libraries then you should look there too.