At the moment there is no option to specify gMSA account as ExecuteAsCredential. Attempt to do that (by passing credential w/o password) will result in error:

The password supplied to the Desired State Configuration resource MSFT_xScheduledTask is not valid. The password cannot be null or empty.

Following instructions here it should be possible with the command used inside this resource:
https://blogs.technet.microsoft.com/askpfeplat/2012/12/16/windows-server-2012-group-managed-service-accounts/

The main difference I see is that this instructions suggest to use New-ScheduledTaskPrincipal and pass object created by it to -Principal parameter on Register-ScheduleTask command.

As this looks like a different code path from existing one I think it would be best to support it in separate parameter (mutually exclusive with ExecuteAsCredential), but would like to first verify this is not already planned to be addressed in a different manner.

As a result we can't use the resource to configure large portion of scheduled tasks that require gMSA account to run correctly.