I want to install Linux Mint alongside Windows 7 and Gentoo. I've already installed GRUB along with Gentoo and written the grub.conf file. Is there any way to stop the Mint GUI installer installing GRUB again?
No, there is no such selection of 'Not installing grub'.
However, what you can do is simply select it to install grub onto its root partition. Then you pass control to your master boot loader, in the case your Gentoo, run from Gentoo, and do a update-grub to get Mint into its grub menu.
The Ubuntu/Mint installer does not allow no Grub install. This can be very annoying and I don't understand why. Other distros do, such as SuSE. A work-around is to install the new Grub to the linux partition itself and leave your existing one in the MBR untouched. Note, one should not use the Grub in the partition because it is unreliable due to the lack of boot-loader provision in the ext format.