It doesn't matter if it is a berm, split level, ranch, 2 story... GLA is not based upon the grade. 99.9% of the time, the GLA will start on the kitchen level (Sorry, not sorry, ANSI). Below that will be the areas that can be finished and function as a finished basement of a typical ranch or 2 story. It'll be a rec/family room, extra bedrooms, extra baths, utility room, etc.
You tell the GLA by the inside of the home with all the windows covered; you won't know where the grade starts... it may be typical, it may be built into a hill, or it could be completely under ground or hanging in a tree. If it strays from the typical placement in the dirt, then you make an exception from ANSI standards. It's all about function and keeping the comparison on an equal level do that your appraisal is done in a way that is simple to understand and produces the least amount of adjustments.