There are a number of factors that potentially affect the GI of breads including:
The type of flour (wheat, rye, barley, etc)
The amount of each type of starch in the flour (i.e., ratio of amylose to amylopectin)
The method of milling the flour (i.e., steel vs stone)
The addition of other ingredients (seeds, kibbled grains, dried fruit, sugar, fibre, water, etc)
The fermentation process (yeast and proofing time)
How the bread is cooked