This article is from the Amiga FAQ, by Ignaz Kellerer with numerous contributions by others.
Any Amiga (ranging from A1000 upto A4000/40) will run amigados-gnu utilities. A minimum of 4MB memory is needed in order to compile small/medium projects. More memory will be needed for large projects, such as recompiling gcc itself, or programming C++. Gigamem and VMM do work with GCC so *maybe* less memory will work. But in this case, an MMU equipped Amiga (A3000,A4000/40) is necessary. See 68EC0xx.
A full installation, including C++/ObjC related files, inline headers, cbm-headers, (see Includes) roughly requires about 20 MB of HD space.
Support for 1.2/1.3 is dropped. If you have 1.2/1.3, get a better Kickstart version. Gcc works under KS 1.2/1.3, but the full functionality is only available under KS 2.x+. A fast CPU (eg. 68030@25MHz or better) is also of help.