What value type is a serialized GUID?

I'm doing some very dirty editor stuff by getting/settings serialized information on assets that don't have API's or any exposed classes, specifically the OcclusionCullingData assets.

There's a SerializedProperty in there called "scene" and it has the GUID type, I think containing a reference to a scene asset. However I can't find what kind of value type the SerializedProperty is. It's exposed in the inspector as an array of numbers, presumably a byte array? I'm not sure how to get or set it properly from script.

Anyone know how Unity serializes/deserializes GUID references and if there is a helper class to do so cleanly?