You have a couple of different options here:
SAP has a number of legacy methods available broadly described under SeCATT and these cover the SAP GUI testing.
Additionally there is the Solution Manager (SOLMAN) integrated testing workbench which can be used together with HP's QTP/Quality Center as part of SAP TAO
The advantage of using the SOLMAN approach is that in theory you can identify an issue or bug in the SOLMAN service desk, promote this to a development fix, have it fixed in SOLMAN, promoted to QA, tested in QA and deployed in PROD with the full lifecycle documented and assured. That's the theory at least.
QTP has the benefit of being suitable for more than just SAP GUI testing too including portal and browser based applications.
Many SAP customers would choose a hybrid approach using a combination of tools and I only know of a few that have committed to just using QTP for everything.
You can also look at the offerings of Borland but I don't know of anyone using these for SAP