Is there a way or work around to sync THINGS gtd app with any calendar app (Mac's Calendar/ Google calendar/Fantastical / Calendar 5) so that I can see all the reminders and events in one place?
Very good question!
I used Things for about a year, and never found a way to sync with iCal or any other.
I now use Goodtask, it syncs with both iCal and Reminders.