Expected, Gimp got removed from Debian testing and thus also not available anymore in Kali:

https://tracker.debian.org/news/1569365/gimp-removed-from-testing/

see I HAVE SOLVED THE ISSUE
Type "sudo nano /etc/apt/sources.list "on your kali linux bash
then after a file will open then just add this "deb http://deb.debian.org/debian/ stable main contrib non-free " after adding this content just control x and y and save the file don't change any content inside the file just add this
after that sudo apt update
then sudo apt install gimp
IT WILL WORK

Adding the sources of another version / distribution is usually not the best idea.

Adding the sources of another version / distribution is usually not the best idea.

Indeed, mixing Debian repo and Kali repo is likely to bring trouble, unless you know very well what you're doing. Better install Flatpak and install gimp from there, I'd say.

In the meantime, I imported the gimp package from Debian unstable into Kali, it should be in kali-rolling in a few hours if all goes well.

Here is one solution. This sets apt's priority to the Kali repository first. If anything is missing, the Debian repository is used.

$ cat /etc/apt/sources.list
# See https://www.kali.org/docs/general-use/kali-linux-sources-list-repositories/
deb http://http.kali.org/kali kali-rolling main contrib non-free non-free-firmware
deb-src http://http.kali.org/kali kali-rolling main contrib non-free non-free-firmware

deb http://deb.debian.org/debian bookworm main contrib non-free
deb-src http://deb.debian.org/debian bookworm main contrib non-free
deb http://deb.debian.org/debian bookworm-updates main contrib non-free
deb-src http://deb.debian.org/debian bookworm-updates main contrib non-free
deb http://security.debian.org/debian-security bookworm-security main contrib non-free
deb-src http://security.debian.org/debian-security bookworm-security main contrib non-free

$ cat /etc/apt/
apt.conf.d/ listchanges.conf sources.list trusted.gpg.d/
auth.conf.d/ listchanges.conf.d/ sources.list~
keyrings/ preferences.d/ sources.list.d/
user@linux:~$ cat /etc/apt/preferences.d/00priority
Package: *
Pin: origin http.kali.org
Pin-Priority: 500

Package: *
Pin: origin deb.debian.org
Pin-Priority: 1

Package: *
Pin: origin security.debian.org
Pin-Priority: 1

$ # the gpg-keys were already installed
$ find /etc/apt/trusted.gpg.d/ | sort
/etc/apt/trusted.gpg.d/
/etc/apt/trusted.gpg.d/debian-archive-bookworm-automatic.asc
/etc/apt/trusted.gpg.d/debian-archive-bookworm-security-automatic.asc
/etc/apt/trusted.gpg.d/debian-archive-bookworm-stable.asc
/etc/apt/trusted.gpg.d/debian-archive-bullseye-automatic.asc
/etc/apt/trusted.gpg.d/debian-archive-bullseye-security-automatic.asc
/etc/apt/trusted.gpg.d/debian-archive-bullseye-stable.asc
/etc/apt/trusted.gpg.d/debian-archive-buster-automatic.asc
/etc/apt/trusted.gpg.d/debian-archive-buster-security-automatic.asc
/etc/apt/trusted.gpg.d/debian-archive-buster-stable.asc
/etc/apt/trusted.gpg.d/kali-archive-keyring.gpg

@mrpn IMHO still dangerous to mention like this as a possible solution, Kali users are often not tech affirm and could break their system if not correctly applied.

Closing as the package is back in kali-rolling