I'm not sure myself of the GPS tolerances but you could reach out to Trimble or their competitors to ask what type of slopes their equipment is able to monitor. That would give you some independent info to use.
If its critical to know this for your concrete slab, you may need to do some investigation. If its only shallow information that you need, perhaps test pits instead of drill holes and ideally outside of the building footprint so you don't disturb the area you are building on.
