Independently of whether you like the treatment of the theory by A (maybe it's incomplete, low-quality or even partially wrong), you have come across the theory through A. At this point, it is a matter of integrity to give credit to A for this.
Yes, the theory has been around before, and OP can develop their own derivatives, that is perfectly fine. It is ...
Honestly, there's too less info provided here for strangers on the internet to guess and come to a conclusion. I'd recommend talking to your research guides to look into the content and sort out the matter. After-all, it's a few definitions, and you have branched off to create fresh research on your own anyway, so your research isn't in danger. "Pick and ...
Only the last option applies.
Once you assign all rights to the IEEE (or anyone), then you no longer have the right to license the work yourself.
And the IEEE statements is pretty clear that they retain almost all rights. It doesn't mean that they might not give a more permissive license at some point, but you no longer have that ability.
As I haven't seen either code, and as a university professor myself, based on what you have mentioned, I would recommend that you consult with the GPL licensing authority.... They can decide about the proper respond both legally and ethically
What that person did, was not just plagiarism, but also copyright infringement. They took a GPL licensed work (legal), modified it (still legal), published it, but not under the GPL license (copyright infringement), and claimed it as their own work (plagiarism).
If you wanted to be nasty, you could tell the original author of the code about this, and if ...
With the caveat that I am not a lawyer, the answer is "no".
code A, which is GPL-licensed
code B, which is a derivative work of A and not GPL-licensed.
The GPL license requires that all derivative works are also GPL-licensed. As B does not carry the GPL license, it is a copyright violation and (depending on jurisdiction) it is probably illegal ...