GMinder trys to ping www.google.com prior to downloading events. If ECHO is disabled on your network (possibly blocked by your admin), then events will not be downloaded even if your calendars can be downloaded. With GMinder v1.2.8, an option has been added to enable/disable the connectivity test. To skip the ping, go into Options and uncheck “Test connectivity before downloading events”.
EDIT: Version 1.2.8 used some faulty logic when deciding if it should ping. It has been quickly replaced by 1.2.9. I apologize for the mistake, please download the new version.