I want to save to a text file the CPU and memory usage real-time info displayed in Gnome System Monitor 3.6. Does anybody know if this is possible? If so, how can it be done? If it's not possible with Gnome System Monitor another tool is fine, as long as it works.
BTW, I'm not afraid to work with the terminal here :D
Use atop. You will have to install it with your favourite package manager (you need "universe" enabled).
Although it is primarily an interactive monitoring tool (for the command line) you can also use it to store the data in a file and later read the from this file.
Please read the man page for atop and find useful examples way down (at about 98% through the page)
You could periodically pipe the output of a call to vmstat (in a terminal) into a text file.
What are the specific statistics you are looking for?
You can also install mp stat (sudo apt-get install sysstat) and then run
(mpstat -P ALL; echo; free) > sysinfo.txt
