There's no definitive answer until someone purchases one. None of the developers have one. You would have to buy one yourself to test it (which would then serve as the test, already done, for the others to look at before buying their own 128 GB cards).
Acting on purely theoretical information here:A Microsoft Knowledge Base article
suggests that Windows may be unable to operate on FAT32 volumes exeeding 127.5 GiB in size. So it may be that, while the GCW Zero can accept 128 GB cards, your computer, if running Windows, won't even be able to write to it.
However, a blog article on Dedoimedo
suggests that using a Linux live CD to format a 931.5 GB hard drive to FAT32 creates a partition that can be read and written on Windows.
Regarding higher-capacity SD cards and microSD cards specifically, the host (computer or GCW Zero) must be able to accept the extended sector addresses in the SDXC (Secure Digital eXtended Capacity) protocol in order to correctly write to the card's entire capacity. A few users have tested 64 GB cards, which are officially SDXC, so 128 GB cards should work by extension.
Ultimately the decision is in your hands:
- have a well-tested 64 GB or 32 GB card in your GCW Zero usable immediately; -or-
- have a 128 GB which you would be the first one to test in this community. If it works, congratulations, you have a ridiculous amount of storage. If it doesn't work, you get to warn the community of this fact, and your card might get supported in a future version of upstream Linux.