Yagarto has a Mac build for gcc-4.6.2, released October 2011
gcc-4.6 is the first gcc which claims to support Cortex-M4
GCC now supports the Cortex-M4 processor implementing the v7-em version of the architecture using the option -mcpu=cortex-m4.
Scheduling descriptions for the Cortex-M4, the Neon and the floating point units of the Cortex-A9 and a pipeline description for the Cortex-A5 have been added.
The current Maple IDE uses gcc-4.4.1, released July 2009
So you might prefer the IDE now, but presumably you haven't really stressed or fully explored the STM32F4?
Of course, the STM32F4 will run Cortex-M3 code - Cortex-M3 is a subset of Cortex-M4, but there are a lot of useful extra instructions for Cortex-M4 including DSP (MAC and fused MAC) and some vector (SIMD) arithmetic (not NEON).
Years ago, I used gcc to get at the old Apple Mac PowerPC AltiVec SIMD hardware, and it worked reasonably well, at first with intrinsics, later with gcc doing more work. So I'd like to experiment with STM32F4 SIMD support, and DSP. I'm hoping gcc has that type of support, without me having to do it all by hand in assembler.