The main difference between the Gentoo install and those of Slackware or Debian is that Gentoo does not have an installer program that guides you through the process. So, you have to manually partition your drives, compile your kernel, etc. It is a more involved and time consuming installation, but Gentoo's install documentation is excellent. You should have a relatively easy time if you're an experienced Linux user who has a general idea of the steps necessary to setup Linux. If it's your first time, then Gentoo might be rather daunting.
The Debian installer, in my experience, isn't much better when to comes to user friendliness. For example, I once attempted to install on a partition that wasn't large enough. As the installer copied files, it suddenly stopped, and informed me that the installer had just exited with a return value of 1. Not very intuitive...
Generally speaking, I wouldn't recommend Debian or Gentoo for one's first Linux system. It tends to be better to use a simplier distro to get to know Linux better. Once you have that knowledge and experience, the more customizable distros like Gentoo and Debian start looking very attractive.