In Google Classroom the assignments allow you to set the points possible. The drop down menu gives you choices of 1, 20, 50, 100 or Ungraded. These options allow you to quickly assign a point value, but you are not locked into those choices. Double click on the GradePoints box to enter a custom integer value (sorry, no decimals.)
When you start to enter a score for a student the points you enter for a student are shown out of the points possible. If you change the points possible in the GradePoints box this value is updated for all students. You will notice the denominator changes for each student.
You are able to give more than the assignment point values if you want to give a student extra points. If the assignment is worth 20 points you can give a student 25 points.