It doesn't work like that. Code written for a (serial) CPU would almost always run slower, if at all, on a GPU even if perfect emulation was available (it isn't, afaik). The processing code needs to be written specifically for the purpose of parallel programming. Multicore CPUs are starting to get people thinking in that direction, but you can't just summon "GPU magic" and get everything instantly working.
From a quick look online, experimental libraries like C++ AMP
appear to be simplifying things and making GPU compute available from a standard C++ application, but you need to use the data structures and language macros defined by the library, it cannot just be bolted onto code and be expected to work out of the box.