Can reptiles digest grains?
This is not a yes / no question. Many tortoise eat plants (veggies and fruits) while some turtles eat fish too. Snakes are basically predators and do not dwell on plants neither do most of the lizards. I think if a reptile is capable of digesting plant food in general, then it should be able to digest preprocessed grains as well. 

Edit:  As for whole intact grains I'm unsure, as canadianer pointed out in comments section, intact grains are hard(er) to digest. /end of edit/
I found this article where scientist fed alligators with a mixed diet in various setup. The diet included corn too and the 'gators responded well in terms that they could use carbon-hydrates.


  Recently, we (3) have demonstrated that hatchling alligators benefit from and apparently do utilize some dietary carbohydrate.