Garden of Brilliance is a mysterious thing mentioned by both RENE and Reverse ÜLAR in Mr. Rainer's Solve-It Service.
RENE states that, although he could've made the Janitors, they were gifted to him from the Garden of Brilliance. In another route, Reverse ÜLAR tries to convince AIK to look into ÜLAR's raincoat by telling him that it came from the Garden of Brilliance. This piques AIK's interest in the subject, who reluctantly takes the raincoat to research it.
Much is unknown about the Garden of Brilliance. It is unknown if it is an organization or a place, and the significance of the Garden of Brilliance is also unknown.