Hi Sophia,
As I understand your client is asking for GAP analysis, and he want to know the benchmark for SAP HCM. With that result he want to understand the what is missing in present system.
The first suggestion you go through the pain points of Client (why they feel that present system is not upto the mark) understand the entire client process, as Mr. Siddharth said it a big area as per client requirement we have to implement.
The second thing is take a note of all manual affert that they are using in the area of Personnel administration. once you understand where is the pain then you can give solutions for each and every pain point 
Good luck, feel free to revert with your doubts.
Regards,
Naresh.KM