This should be easy to diagnose, given the conditions the OP states ...
As quickly as the oil is gaining volume, if this were a fuel dilution issue, the oil would REEK of gasoline. Further, the fuel efficiency would have seen a sigificant negative shift. If these two things are present, then it's a fuel leak (pump or injector). If the fuel does not have a very strong gas odor, then move to the next issue.
It could be an internal coolant leak. But again, as much volume and as quickly as the OP states the oil level changes, it would have to be a really, really bad leak, and he'd see coolant volume loss in the degas tank as well as see coolant vapors in the exhaust. IF those are present, deal with the coolant issue.
I suspect this is a mistaken oil volume issue ... as discussed in other posts above. Given this is the most likely possibility, verify with a GM dealer the proper dipstick part number is in use, and then adjust accordingly. Take the level measurement after a good warmup and proper drainback; don't do this after just a few minutes with oil that isn't fully up to temp.