There are two ways I know to do it, CalSync and AweSync.
CalSync – Free.
“This is a one-way sync program that will sync your Lotus Notes Calendar to your Google Calendar. No installation is required. Includes many options such as automatic syncing at specified intervals. Requires .NET Framework 2.0 to be installed. ”
The good point about this program, is that it is free. My biggest niggle with the program is that I can’t get CalSync to remember the default Google calendar settings.
Note that CalSync only syncs from Lotus Notes to Google Calendar (ie. it’s a one way sync).
AweSync – $20 US
”Synchronize your Google Calendars, Contacts and Tasks with Lotus Notes”
AweSync does two way sync between Lotus Notes Calendar, Tasks & Contacts with Google Apps (including Calendar obviously). AweSync has a Configuration Wizard which walks you though the setup process. I like AweSync. The user interface is clean and well thought out.
My only complaint is that your AweSync product registration is locked to your Lotus Notes user name. So if you change companies, you’ll change your Lotus Notes user name. And then be up for another license. (in comments, the AweSync Dev Team says they’d issue you a new license key for free.)
After evaluating it for 30 days, I was so happy with AweSync that I purchased a copy.