If you entered your Google account credentials correctly but keep receiving login errors, please engage with the following:
Google has enabled an authorization policy for 3rd party applications. What this means is that it will send an e-mail the first time you attempt to log to GTalk through Overwolf. This email will have a link to allow Overwolf access to your GTalk.
You need to give Overwolf permission there and from then on, you should no longer see this error.
One of google’s additional security methods is to prevent non-google apps from accessing your google account. In order to allow the Overwolf platform to use google information, you will have to “Allow less secure apps”, from the Sign-in & security page in your google account.