GoldED is basically your text editing engine. You need a compiler to do the compilation work.
You could get hold of gcc. It's free and is the best C/C++ free compiler for amiga.
For C, there's also VBCC, again free, which supports some of the ISO 1999 standard. I used it for some PPC coding and its pretty good.
As for A1 compatibility, AFAIK, gcc is being used for development and is probably the way to go.
OTOH, if you want just to learn the language (and not get bogged down with CLI compilers), you could just get the 2.1 dev CD. It comes with free StormC v3. I bitch about it all the time (its not a good C++ compiler) but it has a very comfortable user interface. Not much use for your A1 though :-(