Sure, we all know the instagib hits and takes 1000 health off you, that's why it gibs everything except those that have more health. However, there is one sure fire way to survive getting shot by the thing.
Asbestos armor.
If you can find it, which isn't easy, you can survive multiple shots from an instagib. It can't hurt the wearer of asbestos armor at all.
An annoying fact I found out with my little friend here... I don't play with friendly fire on when I got instagibs going. Makes it more fun!