Here is some more info (I had little time when I sent the previous reply).
ZdcSecurePOP3Client is normally in your image, else it can be found in Zodiac-Extra (
http://mc.stfx.eu/Zodiac) and should work for GMail, for example:
ZdcSecurePOP3Client
retrieveMessagesFromGMailAccount: '<your-name>@gmail.com'
password: '<your-password>'
limit: 5.
Reading a multipart message goes like this:
'/Users/sven/Desktop/Test.eml' asFileReference readStreamDo: [ :stream |
stream binary.
ZnMimePart readFrom: stream ].
Test.eml is a message that I sent to myself, containing an image. I just saved it from Apple Mail as raw text.
The result should be something like the following screenshot:

Note the nesting of mime parts.
Please do upgrade to Zinc #bleedingEdge as I had to make a couple of fixes to get mime part support up to date.
Good luck !
Sven