The command you're supposed to use (in more recent releases of linux/gtk) is pkg-config, not gtk-config. gtk-config is intended for pre 2.0 gtk development.
Consider the file you're compiling is called foo.c, to compile it under gtk-2.0, you would use, from the command line the command:
gcc `pkg-config --cflags glib-2.0 gtk+-2.0` foo.c -o foo `pkg-config --libs glib-2.0 gtk+-2.0`
This should compile, and give you a file foo, that can be executed.
but really, use a makefile, as this stuff is a pain to keep typing. I would write out a sample makefile, but there are rules that need to be followed in the formatting of them that makes it difficult to type in the editor window.
# Sample Makefile
CFLAGS := $(shell pkg-config --cflags glib-2.0 gtk+-2.0)
LDFLAGS := $(shell pkg-config --libs glib-2.0 gtk+-2.0)
foo: foo.c
<TAB HERE NOT SPACES>$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
This defines a simple rule saying to make foo, it depends on foo.c, so of foo.c is newer than foo, it will be rebuilt. Where I write 'TAB HERE NOT SPACES' it must be a tab character, and cannot be a set of space characters.