I have installed Windows, Slackwareon /dev/sda6 and Debian on /dev/sda7, GRUB is responsible for loading choosen OS. How to remove Slackware and install Gentoo on /dev/sda6 and not to moving Windows, Debian and GRUB? I read manual at official site but it is for single instalation ?
Does anybody have list of commands for this ?

So you have:

sda6 - 1. Windows
2. Slackware

sda7 - 1. Debian

What you want is:

sda6 - 1. Windows
2. Gentoo

sda7 - 1. Debian

Correct?

The simplest way to do that is to install Gentoo and delete the Debian partition during the install and update GRUB with the proper entry.

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Not sure what you are asking. Are you asking how to add gentoo to grub, or how to install gentoo from debian or using a cd? Gentoo has excellant documentation on how to install either way. Documentation on how to install from another distro is under alternate installation methods. What you basically need to do is format the partiton that you want to put gentoo on, download and put the gentoo files on the partition, untar the files, chroot into the partition and setup Gentoo, all done according to the installation doumentation. If grub was installed by debian, then after installing gentoo update-grub should pick gentoo up

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