GNUstep GUI framework permits to have basic icons support. More complex GNUstep icons support is based on the IconKit framework. IconKit handles icons either vectorial or bitmapped (with size varying from 16*16 to 64*64) with eight bits alpha channel, and by using antialiasing and interpolation IconKit is able to scale icons smoothly. IconKit permits other advanced things like on the fly icons badging/compositing.
It is advised to use the IconKit framework, as it will help you creating your icons and managing them.