Otus and EasyClass. Google Classroom also has a simple gradebook available to teachers.
My recommendation of Otus and EasyClass come with the caveat that you should make a gradebook selection in conjunction with your school's IT director so that you're both on the same page in terms of tech support and compliance with local student data security regulations. And if your school has an LMS that they want you to use, you should use that instead of venturing out to use a different system on your own.
Otus is designed to be a complete LMS (learning management system) for teachers. In addition to the gradebook you have options for sharing assignments and delivering quizzes directly to your students' inboxes. Otus provides teachers with a library of instructional resources (videos, texts, interactive review activities) that they can share with their students. Third-party resource providers are integrated into the Otus LMS. A couple of notable third party applications are Khan Academy and OpenEd. OpenEd offers thousands of math and language arts practice assessments aligned to Common Core standards. Otus is free for individual teachers to use. Otus can also be purchased for district-wide implementation which includes additional reporting tools for administrators.
EasyClass is also intended to be a complete LMS for teachers. On Easyclass you can host classroom discussions, post assignments, post resources for students, and deliver online quizzes to your students. Easyclass discussions and assignments allow you to post notes to the whole class. Assignment notes include due dates. Discussion notes don't include due dates. Both types of notes can include file attachments and links. Through the quizzes option in Easyclass, you can create multiple choice, true/ false, or essay quizzes. Pictures can be included with your quiz questions. You can make instant feedback available to students at the end of the quiz (except for essay questions). Quiz scores can be sent directly to your Easyclass gradebook.