MIPS doesn't require that you install a bootloader. The MIPS PROM handles the booting, but you need to install your kernel as a viable option.

First, install dvhtool, needed to copy the kernel to the SGI Volume Header.

# emerge dvhtool

Now copy over the kernel to the SGI Volume Header. By default dvhtool assumes that the SGI Volume Header is on /dev/sda. If this is not the case (for instance when IRIX is installed on /dev/sda and Gentoo/MIPS on /dev/sdb) you need to inform dvhtool about it using -d <device>.

# dvhtool --unix-to-vh <kernel name in /boot> <name you want to give in volume header>

If you want to see all available entries in the volume, use --print-volume-directory:

# dvhtool --print-volume-directory

To set up your system to boot Gentoo/MIPS you need to go tweak some settings in the MIPS PROM.

Continue with Rebooting the System.

Exit the chrooted environment and unmount all mounted partitions. Then type in that one magical command you have been waiting for: reboot.

# exit
cdimage ~# cd
cdimage ~# umount /mnt/gentoo/boot /mnt/gentoo/proc /mnt/gentoo
cdimage ~# reboot

When you are rebooted, go to the System Maintenance Menu and select Enter Command Monitor (5). If you want to test your new Gentoo installation, you can just run boot -f <kernel name>. To have your system permanently boot into the Gentoo installation, you need to set some variables in the MIPS PROM:

1) Start System
2) Install System Software
3) Run Diagnostics
4) Recover System
5) Enter Command Monitor

Option? 5
Command Monitor.  Type "exit" to return to the menu.

(<root device> = Gentoo's root partition, e.g. /dev/sda3)
>> setenv OSLoadPartition <root device>

(To list the available kernels, type "ls")
>> setenv OSLoader <kernel name>
>> setenv OSLoadFilename <kernel name>

(Declare the kernel parameters you want to pass)
>> setenv OSLoadOptions <kernel parameters>

(Provide the location of the Volume Header)
>> setenv SystemPartition scsi(0)disk(1)rdisk(0)partition(8)

(Automatically boot Gentoo)
>> setenv AutoLoad Yes

(Set the timezone)
>> setenv TimeZone EST5EDT

(Use the serial console - graphic adapter users should have "g" instead of "d1" (one))
>> setenv console d1

Now you're ready to enjoy Gentoo! Boot in your Gentoo installation and finish up with Finalizing your Gentoo Installation.