There are differences between those parts and they can be significant.
Because of it's RAM based architecture the GD variant for example needs a lot of time to start up.
It can also run on higher clock frequencies, and maybe with lower waitstate settings.
If you get the waitstates wrong, a part runs unreliably,(Possibly temperature dependent) or not at all.
One of the variants, I think the MM can run on 5V.
They may be functionally and pin compatible, but there may be differences in I/O drive strength, power consumption (in different modes) ADC noise / linearity and many other details. And those details may either be irrelevant to you, or completely break your project. There are devils lurking there.
For a hand full of breadboard projects with "Blue pill" clones you can experiment a bit, but if you want to use batches of 1000+ there simply is no room for messing about and such decisions can not be taken lightly.
I have no interest with fiddling with the different variants, and therefore bought a batch of WeAct STM32F411 boards. They cost around EUR7 each, but higher clock frequency, genuine ARM parts, more RAM & FLASH, built in floating point, less hardware bugs (apparently the old STM32F103 has quite some), you can solder on a WS25Qxxx Flash chip for a few dimes and you give off a signal that you're willing to pay a bit more if you get decent quality parts.
If you buy GD, CKS, MM or whatever variants, that's up to you.
However, if you buy STM blue pills, and they turn out to be some other chip, then always give negative feedback and complain. There is simply no excuse whatever for it. Some of the "blue pills" do state which variant they use. I'm OK with that. You get what you buy.
Rebranding chips is nothing else but cheating, fraud and a blatant lie. Fiddlling with compiler settings and/or programmer ID's just to get a chip working is a waste of your own time, and their profit if you fall for it.
In my book, it's perfectly all right to punch them back in the face. Start a complaint procedure. Demand your money back because the parts are rebranded and not as ordered. Do this even if you know in advance that the chips are likely rebranded / fake. Make a sport out of it to try to get as many as you can for free because of their fraud of relabeling. Hit them in their wallet where it hurts and do it repeatedly until they stop these fraudulent practices.
Also, have a look at LCSC.com. They have the datasheets for all (or at least most of) the variants.
Even if you only use STM parts, the overall compatibility may make some details easier to understand if you read them in one of the other datasheets. Some variants have separate datasheets in English and Chinese, some are Chinese only, that's a simple immediate stop for me. Using any IC over the complexity of .. let's say a HX711 (which is sorta fun to play with) without an English datasheet is simply not acceptable for me.
I once spent half a day replacing individual chinese sentences and paragraphs in a .pdf with translated strings from some internet website. It's doable, and it works, but it is a huge effort, mostly because of the horrible way's .pdf files are formatted. The inventor of the .pdf format should have been shot 20 years ago.
(Ah well, maybe not that bad. It's not his fault that the .pdf format became a defacto standard and is being abused in horrible ways for which it was never intended.
It was designed as a format that could be sent to a printer, and to get back what you sent to the printer company exactly as you sent it. The format was never designed for distributing information such as datasheets, or reflowing on smallish E-readers and tablets).
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