What are you using now Lilo or Grub? Either should boot it just fine if you've configured them properly. I have never heard of GAG much less used it, so I can't help you there. I've never heard of or used Smart Boot Manager either. It looks like your going to have to Live CD it, reinstall GRUB, and then set it up to boot all, what three? OSes on your computer. (Or LILO if thats your bag)
Thats what I would do if I were you. I'm sure there are other options. You should probably start a new thread if you want your problem to get its due attention.