As you probably know, Google Calendar doesn't have one-click support for ICS files. (Gcal does understand the ICS format, but ICS files need to be manually imported into the calendar, which is a step too far for a lot of people.) But for a long time, you've been able to build special links — which are, in a lot of ways, like Agical links — using a format like:

    http://www.google.com/calendar/event?dates=20160915T100000Z/20160915…


Typically you'd link an image like in your email. Google Calendar will automatically open a window with the event info filled in, and the lead just needs to click Save to add the event.

You could already add those URLs to Marketo emails before Agical came out, of course. But if you're now using Agical links to generate full-featured ICS files, the URL formats are vastly different, easily leading to confusion.

format — response format, set to format=gcal to send the lead to Google Calendar instead of opening an ICS file
Just append &format=gcal to the end of a working Agical link and it turns into a Gcal link. You don't have to set any Google-specific values: Agical takes care of the translation for you.
Note that Gcal links do support Agical's recur and allday options but don't support reminder. That doesn't mean the lead won't get a reminder, but it will use their default time instead of whatever's in the link.