Standard Removing of RecGTasks
If you installed app using Chrome Store follow instructions from official Google support page otherwise
open app page
scroll down to the bottom of the page
click "Uninstall Triggers" button
Manual Removing of RecGTasks
If you, are receiving notifications from Google even after uninstalling the triggers as described above, you can check and uninstall triggers manually.
open the list of Google Apps Script triggers associated with your account using this page https://script.google.com/home/triggers
2. Right click on each trigger from project "Recurrent Tasks for GTasks" and click "Delete trigger"
Depending of permissions you gave to the app you might not see the trigger details. Still you can right click and remove both triggers.