It is currently in beta, go to the settings, search for Linux and turn the feature on.
Then you will have a Linux terminal where you can navigate to your GD folder and run using terminal commands.
chmod +x ./gdevelop
However, you can already copy/move files between Chrome/Linux/Android container using the Chrome file browser. Both Android and Linux container has an entry point in the Chrome file browser where you can access files that you or the apps you are using has been created in the container and you can freely copy and move them to anywhere you want. If you want to access the actual Linux and Android file system you can install a Linux and Android file browser, in case of Linux you can also use the terminal. But accessing the Chrome and Android file system from a Linux app and vice-versa never going to happen on Chrome OS at least not by default. All 3 file system will be isolated from each other for good.
./gdevelop, I receive the following error:
./gdevelop: error while loading shared libraries: libnss3.so: cannot open shared object file: No such file or directory
All I did was unzip the tar.gz to the folder. Am I missing something?
cannot open shared object file: No such file or directory
You need to install the missing library libnss3. In case you are not familiar with the Linux terminal I recommend to install synaptic.
sudo apt-get install synaptic
After you should have a new application to launch called Synaptic. This application allow you to search, browse and install libraries using a GUI.