You can express any number type in pure lambda calculus.
But that is not what this is testing. And I’m not even sure which number system the mentioned roots of unity are in. Maybe it’s supposed to be generic over number systems in a delightfully untyped lambda calculus sort of way?
edit: after pasting the entire problem including the solution, ChatGPT is able to explain “GN numbers”. I suspect its explanation is sort of correct. But I get no web search results for GN numbers and I can’t really tell whether ChatGPT is giving a semi-hallucinated description or figuring it out from the problem and its solution or what.