The /newroot folder is made by Gentoo during the boot process. I
Every time you boot it creates this /newroot directory? That's a little different. I read over the page from your Pendrivelinux YUMI link and did a little more reading on grub4dos. Didn't realize they were strictly windows programs. I've booted windows 3-4 times in the last year and never used either of the above programs so I doubt if I'll be any further help. I haven't had any problems booting multiple Linux operating systems on USB/CD/DVD with Legacy Grub so never bothered with other methods.

Good Luck!