This splash support for Gummiboot just comes down to being able to display a BMP image as part of its configuration file when starting the UEFI boot manager otherwise it will fallback to the stock image.
The support landed in Gummiboot on Sunday with this Git commit. While not as widely used as GRUB2, Gummiboot is still under active development and frequently seeing new work by Red Hat's Kay Sievers.
This work is part of Gummiboot v40. Also part of this new release are just other small changes.