You don't need to change the pointer value. The GPU and the CPU have the same page table structures and both use the same pointer representation for "somewhere in common memory".

On the GPU there are additional pointer types for different local memory, e.g. LDS is a uint16_t indexing from zero. But even there you can still have a single pointer to "somewhere" and when you store to it with a single flat addressing instruction the hardware sorts out whether it's pointing to somewhere in GPU stack or somewhere on the CPU.

This works really well for tables of data. It's a bit of a nuisance for code as the function pointer is aimed at somewhere in memory and whether that's to some x86 or to some gcn depends on where you got the pointer from, and jumping to gcn code from within x86 means exactly what it sounds like.