How to write and build a GTK+ app on Ubuntu

$ cat hello.c

#include 
int main(int argc, char *argv[])

{

  GtkWidget *window;

  GtkWidget *label;
  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  label = gtk_label_new("hello, world");

  gtk_container_add(GTK_CONTAINER(window), label);

  gtk_widget_show(label);

  gtk_widget_show(window);
  gtk_main();

  return 0;

}

$ gcc -Wall -g hello.c -o hello `pkg-config --cflags gtk+-2.0` `pkg-config --libs gtk+-2.0`

$ ./hello

pkg-config simply outputs the headers and libraries that need to be passed to gcc.

More information here.