if you want the global variable to be accessed externally, declare
in the .h file with extern > if you only want all functions in the current file to share the global variable, declare

this is not only a problem of global variables, but also involves the use of -sharpinclude . When the compiler sees -sharpinclude , it copies the contents of the specified file completely into this file. In terms of the contents of the three files you give, when compiling main.c , after the compiler processes -sharpinclude "ggg.h" , the main.c file looks like this:

// ggg.c

//ggg.h
-sharpifndef ggg_h
-sharpdefine ggg_h

-sharpinclude <stdio.h>

int num;//
void ggg();

-sharpendif /* ggg_h */

void ggg(){
    
    num =1;
    printf("ggg::%d",num);
}

so there is also a int num variable in ggg.c . When
links the compiled target files of these two files, two int num appear, and the compiler will naturally report an error.