In order to login to ClassGather directly, you'll first need to be logged into your district's LMS. Next, you'll need to navigate to your district's ClassGather instance, which you can do by attempting to visit any page on ClassGather. You'll be redirected to a login screen that looks similar to the one below.
There may be a "Login with [LMS]" button. Clicking that while logged into an LMS integrated with ClassGather, will automatically log you in using your LMS credentials.
Note: If this button is not present on your instance, you will need to contact your system administrator, because it means ClassGather is currently not set up to be integrated with your LMS in this way.