1. Download Apache Tomcat.
  2. Save anywhere.
  3. Open terminal and navigate to tomcat /bin folder.
  4. type chmod 700 *.sh to add executable permissions.
  5. Modify user rights in tomcat-users.xml
  6. startup tomcat via /bin/startup.sh
  7. Open localhost:8080 in firefox
  8. Open Manager App
  9. upload .WAR file of your GWT app
  10. deploy
  11. visit app via localhost:8080/<gwt-app-name>

11 steps which took me a whole day to find. What. the. eff.