I'm not really a GPU person, but my understanding is that GPUs have very simple memory models, memory reads and writes are scheduled more manually. But this is part of their appeal, why they can run so fast, but also why they aren't as general as CPUs.
But my knowledge is a bit out of date, since GPUs are actually including caches now, and what they can do seems to evolve rapidly.
Yeah, the traditional GPU usage is a streaming thing. Textures are read only, vertex and fragment data is either output-only or input-only depending on shader type. The only R/W memory requiring synchronization is the framebuffer, which is handled by specialized hardware anyway.
But my knowledge is a bit out of date, since GPUs are actually including caches now, and what they can do seems to evolve rapidly.