Redmine

This will require building a map between

GCal's calendars and title
Redmine's users and projects
For example, calendar GCal UserX gets mapped to Redmine User X. GCal entries with title ProjectA get mapped to Redmine Project A. Then all entries in GCal that match this are put into the database regardless of current project affiliation (User X may no longer belong to Project A in Redmine, but we don't care)