I had no idea this existed, now I cannot live without it! Todoist can integrate two-way sync with Google Calendar! My one gripe with Todoist is that it couldn’t sync with Google Calendar, and my one gripe with aCalendar+ was that it could only sync with Google Tasks. This left me with no other option but to switch between two separate apps, one for calendar and one for keeping a todo list, which I grant you is a first-world problem, but is still valid.
Google Calendar syncing with Todoist solves this though. The sync now basically comes to:
Todoist <-> Google Calendar / aCalendar+