Guar and Xanthum Gum
These two products can, to a certain extent, act as a replacement for gluten. Each of them will help to hold the mixture together, resulting in a less crumbly gluten free bread.
You would use one or the other, not both, in a recipe.
Xanthum gum is more expensive than guar (unless you live in the UK and can get it on prescription), which may help you choose. Be aware though that it is possible to suffer ill effects from both.
Guar can sometimes cause tummy upsets and is often used to treat constipation! Some people are, however, sensitive to xanthum gum.
Whichever you decide to use, it is important to measure these ingredients carefully. Adding too much can cause your bread to end up slimy! Be sure to incorporate the gum with the flour thoroughly, before adding any liquid, or you could end up with a sticky mess.