> So, if I looked at a function, understood it, then reproduced the function from my grasp (not necessarily verbatim, but by understanding the logical flow), would that be subject to the GPL? What if it varied? How much does it need to vary not to be considered derivative?

That's an interesting question. In general, the answer is no, but could change depending on if you had the function open at the time of writing the new function.

But a lot of us have knowledge of basic data structures, etc, from reading GPL-licensed code (or perhaps even proprietary code in textbooks). So, in general, coming up with your own understanding is fine.