Hi Baji,
If you already know GATP, then it's matter of understanding your clients business and solution implemented. Check if there are any deviations from standard and what kind of solution is implemented. Rest all is on the basis of the agreement between your client and you as service provider i.e. will you support only incidents or will you also support new functionalities and enhancements etc. The main key is you should have good understanding of GATP solution so that you can fulfill your clients requirement in agreed service contract.
For GATP refer below link:
Global Available-to-Promise (Global ATP) - SAP Library
Regards
Rahul