GPC does not support BP's 6-byte Real type. It supports Single, Double and Extended which, at least on the IA32 and some other processors, are compatible to BP.
For BP's 6-byte Real type, GPC's System unit provides an emulation, called BPReal, as well as conversion routines to GPC's Real type (which is the same as Double), called RealToBPReal and BPRealToReal. You'll probably only need them when reading or writing binary files containing values of the BP 6-byte real type. There are no operators (e.g., +) available for BPReal, but since GPC supports operator overloading, you could define them yourself (e.g., convert to Real, do the operation, and convert back). Needless to say that this is very inefficient and should not be done for any serious computations. Better convert your data after reading them from the file and before writing them back, or simply convert your data files once (the other types are more efficient even with BP on any non-prehistoric processor, anyway).