I have never tried to do an imapsync with gmail, but aren’t those folders with Gmail some kind of ‘virtual’ folders? If so, I can imagine that, since they might not be real folders, they can’t be excluded… (not based on evidence, so consider this speculation)
If logs are not disabled in the source code then they might be in /var/log/imapsync.
IIRC, some folder names are localized (they vary according to the Gmail language settings). You can verify them by listing the IMAP folders (fun fact, gmail treats labels as “folders”).
I’ve used imapsync (CLI) with gmail to exclude some folders, but not the Important folder. But have not used the imapsync UI implemented on NethServer.