A Globally Unique Identifier (GUID) consists of 32 hexadecimal characters (https://en.wikipedia.org/wiki/Universally_unique_identifier). Is the chance of a GUID that consists of only digits as simple as 10/16 to the power of 32?
I have a feeling that this is a too simple approach and something like Benford's Law might skew with the results?
No, your answer is correct, assuming the GUIDs are generated by generating each digit independently of these others and uniformly over the digits (which is a reasonable assumption).
Benford's law occurs in situations where the numbers under consideration signify quantities, where larger quantities are less likely. Since the numbers (of a certain fixed length) starting with 1 signify relatively smaller quantities, they are more common, giving rise to the law.