The short answer: GPU computing is possible with the software PTP is built with, but not with the PTP download (due to security reasons). Also to note not every project supports GPU computing.
Explanation: PTP is a customized version of the BOINC software (http://boinc.berkeley.edu). PTP and GridRepublic enable the security features of BOINC to keep a computer safe. Enabling GPU computing requires disabling a security feature which not compatible with GPU computing at this time.
If you join a project that uses GPU computing you would need to install the default BOINC software found at http://boinc.berkeley.edu/download.php. Upon installing making sure the box for protected execution is not selected. BOINC is compatible with the PTP Facebook Application and projects.
Each project is managed and developed independently thus it is up to each project decide whether to create a program for the GPU's to use.
The three projects currently shown by the PTP Facebook application do not support GPU computing, but there are other projects that you can access that do use GPU computing.