The two gents of Team Galag, from Saudi Arabia, have put the ideas and money behind it. This will be their third year running the Gumball, and from the humble beginnings of a Porsche 911 Turbo in 2011 they went on to a Lamborghini Aventador and Mercedes SLS AMG Roadster last year. The Tumbler is being built by Parker Brothers Concepts, the company that constructs vehicles for the SyFy Channel's Dream Machines show.
As for that one-off supercar, it's called the TG1 for the time being and building it has been "a decision that we have regretted ever since." Not that there won't be a TG2, mind you. They've promised updates on the build, so we can safely say that it's not the diamond-trimmed LykanHypersport. Which is a good thing.
If you want to look out for them, the Gumball route runs through Stockholm, Turku (Finland), Helsinki, St. Petersburg, Tallinn, Riga, Warsaw and Vienna on its way to the Monaco Formula One Grand Prix the weekend of May 23.