You added that part afterward in an edit and I was still replying to that part, but you’re right and that’s what rms is encouraging: that we both have the code and both have the freedom to learn, modify, and tinker on it as does anyone we convey the code to.

Only with the consent the author == if my code is GPL, not if I'm copying the code into my proprietary code.

Sure, it’s the L in GPL that gives everyone the right to use GPL code in their product under certain conditions as spelled out in the license.

Nothing in the license precludes the sale of such a combined work though, as in your devil’s advocate query.

I’m not even sure where the confusion or disagreement is in this sub-thread to be honest. The license is short and fairly clear; there’s ample additional “what does this mean for inclusion of GPL code in commercial products?” FAQs and a fairly large body of commercial products openly marketed which include GPL code in them.

[0] - https://www.gnu.org/licenses/gpl-faq.html#GPLCommercially