I have a 98 GTP and I have a chance to by a 99 GTP are they comparable for parts?

Hello, such "parts" swaps are discourage unless cosmetic. Each item you are looking to swap needs to be verified for compatibility. Even a single model year change with little cosmetic differences will likely have different wiring harnesses, software needs, module configurations and even module differences in the first place. If your engine block happens to be the same, perhaps the cylinder head is different. Or the fuel injectors. Or timing control. It is far too murky to know each detail. I will say when comparing, briefly, parts, that the engine assembly appears to be the same from one year to the next. However, the transmission part number does change. This leads me to believe there could be internal component changes and different software controls and things we can't even begin to speculate on. As a result -- there may be some value to acquire a "parts car" like this, but use extreme caution and check each functional component you are looking to swap before doing so.