Gecko Patch File format, or GPF, is a feature of Gecko OS that the internet seems to have overlooked. (A google search yields just short of anything meaningful) This document aims to explain how to use this feature without the need to reverse engineer the source code.
GCT files, which you should be quite familiar with, contain code for Gecko OS to interpret and execute during gameplay. GPF files, on the other hand, simply contain chunks of data for Gecko OS to write out over specified places in memory, before the game boots. Here's how the format describes this information: