Are there libraries for email message parsing (particularly, from Gmail's IMAP server) in Python (except for the standard email library)?
Or if not, maybe there is some C++ library for this purpose (and I'll be able to use it through SWIG)?
This question does not make sense. You can access GMail either using IMAP or POP3.
And for parsing retrieved emails you use the 'email' module of Python. Python provides support for all this out-of-the-box "all batteries included".
python: libraries for parsing email