Import grades to most LMS programs (Schoology, Edmodo, Canvas, Moodle, InfiniteCampus, JMC, Blackboard) by exporting grades for individual assignments or for the whole class as a .csv file. The students email address is used to identify the student between systems.
If you're using Google Classroom you can submit grades directly to the connected Classroom assignment. When you are looking at the grades section of a lesson you assigned to a class that was imported from google Classroom you'll see a "Submit to Classroom" button in the upper right. Grades are submitted as an average score out of 100 points.