I'm not sure, but as gmail requires a secure connection, you will probably need to install sTunnel as well as OpenSSL - you will find everything at
http://www.stunnel.org/download/binaries.html . I have this working very well with another spam filetr - K9 (99.15% accurate over 3 years - download from
http://keir.net/k9.html )
Generally, spam filters will not work with encrypted mail. sTunnel lets you break in to the unencrypted text (in the privacy of your own computer).
You end up with a daisy chain of email client -> Spam Filter -> sTunnel -> eMail Server. I set sTunnel up to listen to ports 11011 and up as these are undesignated. My email client talks to K9 on port 9999 and then K9 talks to sTunnel on 11011 and sTunnel talks to the email server on port 995.
I had a quick look at the user manual for SpamKiller, but there is nothing in there to say which ports SpamKiller uses by default. You will have to contact technical support, or switch to the free programs I've mentioned.
Richard