Reading the comments and concerns here lead me to think there are several misconceptions about GBC. What balls to use, what parts do you need (System, Technic), how complicated does it have to be, etc?
If anyone has questions or concerns that make them wonder whether they can build one, ask it, several GBC builders are here who can help.
There are other GBC contests in the past as well, including one that’s about to start up, in case people are curious as to what those look like.