I think I might be doing it wrong, because I didn't do any looping. I'm a bit too lazy and impatient for that, who wants to spend their afternoon adding all those numbers up even with a computer.
[joe24pack@staropramen ~]$ python
Python 2.6.6 (r266:84292, May 1 2012, 13:52:17)
[GCC 4.4.6 20110731 (Red Hat 4.4.6-3)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> def gauss(x):
... return (x+1)*(x/2)
...
>>> gauss(10)
55
>>> gauss(1000000000)
500000000500000000
>>>