Endless doesn't ship with tools like gcc in the base OS, but you can add them to run/build apps inside a Sandbox using Flatpak. http://flatpak.org/developer.html has detailed developer instructions, but these commands will get you a shell where you can run GCC:
flatpak install flathub org.gnome.Sdk//3.28
flatpak run --devel --command=bash org.gnome.Sdk//3.28
You also may be interested in the Builder IDE for GNOME and other development tools which are now available for download via the app center.