When comparing GTasks vs Doit.im, the Slant community recommends Doit.im for most people. In the question“What are the best to-do list apps for Android?” Doit.im is ranked 32nd while GTasks is ranked 33rd. The most important reason people chose Doit.im is:
The apps borrows some design elements from Material Design-type design, which uses a slide-out menu. Everything is laid out in an easy-to-find manner.
Specs
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Pros

Pro Unlimited subtasks
Unlimited subtasks or sub sub tasks, as deep as you need, excellent for project work.

Pro Synchronizes with Google tasks available natively in GMail
Seamlessly syncs tasks.
Pro dark theme available
Pro time reminder is supported

Pro Closing parent task closes sub tasks

Pro Available on iOS

Pro Works with Chrome's 'Google Tasks Panel' extension
Pro Clean, simple layout
The apps borrows some design elements from Material Design-type design, which uses a slide-out menu. Everything is laid out in an easy-to-find manner.
Pro Daily review which allows for did-take VS should-take time comparison
A built-in daily and weekly review function that lets you compare the estimated time for tasks against the actual time they took, as well as productivity for the day/week.
Pro Low cost
Subscription value is generally lower than other platforms.
Pro Quick add from notifications
Pro Free apps for Android, iOS, Web.
Cons
Con Pop up ads get in the way
The app frequently displays popup ads when you navigate the app, forcing you to pause to close them.
Con No recurring tasks fom completion
Con Does not support attachments
You cannot add attachments to tasks.
Con Support non existing
Con iOS version no longer being updated
iPad upgrades stopped at iOS version 11, so if you want to keep Doit.im it is not possible to upgrade to iOS12. As there is no export facility, you are stuck.
Con Rarely updated
Con Offline sometimes
Con Nonexport and support desk say they will never include it (your data is theirs now)
Con Subscription based
While not expensive, it does cost money to get it for Mac or Windows (although you can view it in the browser for free). The subscription also adds automatic cloud sync, sync with Google calendar, adding tasks via email, manual sort, attaching Evernote into tasks, and more.
The subscription costs $2/month, or $20/year which is very reasonable
Con Sometimes it doesn't see your active subscription
Con Rip off
Con Not very customizable
There is no way for you to create your own "perspective".
