I've used several commercial dev kits that have gcc as the back end. The issue then is what you have to do to satisfy GPL. This includes
1) making the compiler part of the package free - you can only charge for the other bits and/or support.
2) making any changes you make to gcc source available to anyone else.
3) providing a link to and/or distributable version of the source if you changed it.
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4) include a copy of the GPL with your distro
If you used OpenWatcom, you'd have to consult their license, it's OpenSource approved, but not GPL.
I doubt VC2005 Express compilers come under any kind of license that lets you redistribute them.
Also look at DJGPP
http://www.delorie.com/djgpp/ which might be more up-to-date than devc.
<edit>It might, but djgpp is for 32bit DOS. Maybe look at mingw instead,
www.mingw.orgJim