When i enter gnome-system-monitor a window pops up with the CPU % that every process is taking, however it just shows a number like 1,2,3,4... not a real percentage. How can I see real percentages?
Gnome system monitor displays the percentage CPU as a rounded number. For practical purposes, this is more than adequate, and provides you sufficient information as to what is going on. Moreover, this percentage is a snapshot, an averaged measure over a short period of time. In reality, that number changes all the time, so rendering it with a precision of 1% is more than adequate for almost any common purpose.
If, for academic purposes, you want to see the first decimal,
top (installed by default) and
htop (not present on a default Ubuntu install) are your friends. I am not sure of methods or applications that would allow to display that number with even greater precision, and where you could customize the time on which the average measurement is done.