For your information, the specs for the All Trumps high-gluten flour are at http://www.gmflour.com/gmflour/Flour_SpecSheet/AllTrumpsEnrMaltBl%20Br50111.doc.
The specs for the AT flour do not specify the amount of barley malt added to the base flour, but I have read that millers typically add barley malt at the rate of 1-10%.
Barley malt added to flour by millers is diastatic and includes natural alpha-amylase enzymes that act on damaged starch in the flour through a series of successive steps (which I can explain if you'd like) to convert the damaged starch molecules to a simple sugar (glucose), which is then available to, and metabolized by, the yeast as food. This should cause more alcohol and gas production (i.e., a higher rise), and, to the extent any of that sugar remains as residual sugar at the time of baking, as a source of crust color development, and maybe a bit of added flavor and aroma. It is possible to add additional diastatic malt to an existing flour but if too much is added, the water bound up in the dough (the damaged starch can absorb about three times the amount of water than non-damaged starch) can be released by the enzyme activity and yield a slack and wet and sticky dough. Typically, one would add about 1/2 teaspoon to 1 teaspoon of diastatic malt (dry) to three cups of flour (about 0.5-1% of the flour weight). Bob's Red Mill is a common retail source for diastatic malt (http://www.bobsredmill.com/product.php?productid=3529
There is also a nondiastatic form of barley malt, but it does not have any active enzymes. It is used mainly as a sugar for crust color development and flavor. Both forms of malt are available in wet or dry form.
EDIT (4/15/14): For the most recent link to the GM All Trumps flour, see http://professionalbakingsolutions.com/product/all-trumps-enriched-flour-50-lb/50111000