I'm attempting to do the same thing (dual boot Windows 7 and Gentoo) and I am having similar problems.
My GRUB bootloader looks practically identical to yours (except for a splash image)
When I try to load Windows, I get the "BOOTMGR is missing" error.
I'm still messing around with a few ideas on how to make these two operating systems work together, since I'm found next to nothing with Google searches.
I saw this post, and found it relatively useful, so I had to comment.
I wouldn't call myself a Windows expert, nor a Linux guru, but I've got the general idea of what I need to do, and what's going on.
If you guys have any ideas or comments, please feel free to provide constructive criticism!
(I'll try to update this when I find anything new or interesting in this crazyness)
EDIT: Okay, I did a bit of poking around (remember, I'm not really a guru...)
The kind of setup that I had was:
/dev/sda1 = Linux boot partition
/dev/sda2 = Windows 7 partition
/dev/sda3 = Linux swap partition
/dev/sda4 = Linux root partition
When the "BOOTMGR not found" error came up, I decided to reinstall Windows 7, but specifically on the /dev/sda2 partition. (note: When I made a fresh install of Windows 7, leaving a lot of the hard drive unformatted for Gentoo, Windows 7 made two partitions, one for boot, and one for the 100 GBs that I assigned to it)
Of course, installing Windows made it ignore GRUB, so I used the Gentoo minimal install CD to rewrite GRUB to /dev/sda.
the fdisk command says that the Windows partition is bootable (marked with a * in the table) and /dev/sda1 is not.
GRUB still loads, though, and choosing Windows 7 from GRUB successfully loads Windows 7, so I'm not complaining.
Being somewhat of a newb, though, I am unclear as to why this is, and what /dev/sda1 is up to...