I am about to brew a GF beer this weekend as well. I just got my kit in the mail and it came with a nice sheet about the malt and millet from Eckert Malting.
In the documentation it states that their "rice malt has lower diastatic power than barley malt and will require significantly longer mash times to achieve full conversion of starch".
They also state that you will use about 25% more grain by weight (due to size of the grains) to achieve similar wort gravities.
Hope you find this helpful.