I bought & installed GeeTasks. When I tried to enter my Google email&pw in GeeTasks in order to use it, I was refused. After trying & being refused several times, I finally looked at my Google account itself, and found that the login refusal had been logged with my IP address. The reason cited was "less secure" app. I checked security settings and found an option for this reason. It says "Some apps and devices use less secure sign-in technology, which makes your account more vulnerable. You can turn off access for these apps, which we recommend, or turn on access if you want to use them despite the risks." I chose to "Turn On" the "Access for less secure apps". Voila! GeeTasks logged me in, and displayed my default tasks list. Hurrah!
I recommend that GeeTask developer(s) do something about this situation, such as using whatever protocol that Google wants, or at least adding some comment about this feature/bug.