This is a bracket for holding a charging circuit for GameBoy Color. If you are modifying your GameBoy to have a rechargeable battery, this can be useful. You will need to cut an opening in the shell for the USB connector.
It fits a circuit with these dimensions: 27mm x 17mm, such as this one: https://www.amazon.de/gp/aw/d/B07MXYKWVM/ref=yo_ii_img?ie=UTF8&psc=1
I printed it at 0.1mm layer height, but should also be fine at 0.2mm.
Included is the f3d file, in case you'd like to make modifications.