The gameplay programmers usually do not write HLSL shaders - that job falls on the engine programmers or technical artists. I guess another way of looking at it is to ask yourself, "Is this feature game play?" from a gamers perspective, and if it is, then a game programmer usually implements the feature. For example, HLSL is used for programming materials, making cool effects, etc. but usually not something that directly relates to more tangible items such as killing monsters, jumping (wheee), monster behavior, equiping items, shooting guns, role playing design, user interface, earning experience, etc. so you can then narrow down who would work on the feature. At Obsidian we have three types of programmers game play, engine (CTG), or tools (CTG). CTG is the core technology group, and they make our Onyx engine. Engine guys work on "low level" things like rendering, physics, memory management, system IO, file systems, sound/music, animation technology, things closer to the hardware etc. etc. We say game play is "high level".
There's not really anything called the "game stack" to my knowledge. Stacks are a pretty essential tool but it isn't anything more than a list of items that you push and pop from. You can read about a stack here: http://en.wikipedia.org/wiki/Stack_%28abstract_data_type%29