In other words: I let Apple create an iCloud Calendar when enabling iCloud Reminders, but I'm not using the Calendar and I let Apple create an email address but I'm not using it. There seems to be a catch because of the fact that Notes are tied with a email address and Reminders are tied with an Apple iCloud Calendar, but in fact, you can just ignore them and keep using Google for that. In other words : I'm now using iCloud for these two and it works perfectly : very fast and without any trouble whatsoever. After trying a lot of things I just gave up and decided to stick with Google, except for the Reminders and Notes. If you are a Google user like me (Gmail, Google Calendar, Google Contacts) you will probably run into trouble getting the Reminders and Notes working. Now you will have all your GCal events in iCal and all your iCloud Reminders in Reminders syncing seamlessly on both your phone and computer. There will also be 2 iCloud calendars listed called "Home" and "Work" but you can just uncheck those and never use them for anything. The only annoyance is that in iCal you will have to go to Preference (just once) and make sure that your Google Calendar is set as your default calendar so all events will get added to the right place. Since you are only using iCloud for Reminders and are not actually using the calendar anywhere since you have everything configured to sync with Google, you should not have to worry about duplicating events or reminders in any way. Next add a new iCloud account and select Calendars & Reminders. Google Tasks will not actually be anywhere I have no clue why it says Reminders there. This will only Sync your Mail and your Google Calendar, not your Google Tasks.
In System Preferences - Mail, Contacts & Calendars add your Gmail account and select Mail (if you want) and Calendars & Reminders. Then go to Accounts and iCloud and make sure that Calendars is set to OFF but that Reminders is set to ON. Under Accounts in your Gmail make sure that your Google Calendar is set to ON.
In your iPhone go to Settings and Mail, Contacts, Calendars. You cannot sync Google Tasks with Reminders but there is a way to sync everything the way you want it.