how many bytes of GRE overhead does encapsulation add to the original data packet?

The answer is B or 24 bytes.

“P fragmentation issues have become more widespread since IP tunnels have become more widely deployed. The reason that tunnels cause more fragmentation is because the tunnel encapsulation adds “overhead” to the size a packet. For example, adding Generic Router Encapsulation (GRE) adds 24 bytes to a packet, and after this increase the packet may need to be fragmented because it is larger then the outbound MTU. In a later section of this document, you will see examples of the kinds of problems that can arise with tunnels and IP fragmentation.”

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml