The suggested (by ST) GCC platform for the STM32VL-Discovery is the Atollic Suite.
However i have been using Codesourcery-Lite without problems.
You will loose the onboard debugging facility if you doesn't use one of the ST suggested Suites.
But as i'm primarily running Linux , witch ST doesn't support for debugging anyway , it'l make no difference for me. And Codesourcery produces nice code.
I'm looking into doing some "Tricks" with the Value board. :-)
Btw: The Keil supported GCC platform usually is Codesourcery.
Edit: This is a nice page for the Discoveryhttp://gostm32.blogspot.com/
And here is a full intro to STM32 & GCChttp://fun-tech.se/stm32/index.php
I'd still recommend codesourcery lite (The EABI toolchain)