GUI boot is frowned upon for a number of reasons, not the least of which is that it is much slower for multibooters. As Terry was hinting at, the "GUI bootloader" is actually a façade - the bootloader itself is tiny and barebones without the capacity to do much more than show a text-based boot menu. To facilitate a GUI boot menu, the OS actually (partially) boots into the default boot target but before the visual part of Windows starts (higher-level drivers, shell, etc) and the login process starts, it has Windows instead display a menu with the boot options.
If you choose the default boot option (the OS that was already partially loaded), Windows continues booting on its merry way. But if you change the selection, the PC actually reboots into the chosen OS (skipping the bootloader so it looks like it went straight into it), causing it to throw away the time it spent partially loading the first choice.
My memory is hazy on all the details (I can't believe it was 10 years ago almost to the day since Windows 8 was officially released!) but I'm pretty sure that Windows 8 was the first to ship with metro bootloader support (hence the "metro" in the name, as Windows 8 was the first version of Windows to introduce that horrendous desktop UI paradigm) - it's simply not a part of Windows 7. The above description of how the GUI bootloader works should explain why as Terry said (though, in total honesty, I had completely forgotten) the GUI bootloader option doesn't work with Windows 7 as the default choice.