@laura Sorry for the trouble! Unfortunately, this is a limitation that we’re aware of in OmniPlan’s ability to interact with Google calendar accounts. Reading through our notes, it looks like at one point in time Google didn’t support this type of free/busy query, so we disabled this subscribe option for Google accounts. We do have an open request in our development database to re-investigate the feasibility of implementing functionality - I’ll attach your comments so that the rest of the team is aware you’d find the ability to sync free/busy time directly with your Google calendar helpful!
For the time being, you may be able to work around this limitation by configuring your Google calendar in Calendar.app on your Mac. After doing to, you should be able to use the “offtime/overtime from iCal calendar” subscribe option.