There is a much easier way to compile and run C code using GCC, no configuration needed:
Ctrl+Alt+N, or press F1 and then select/type Run Code, or right click the Text Editor and then click Run Code in context menu, the code will be compiled and run, and the output will be shown in the Output Window.Moreover you could update the config in settings.json using different C compilers as you want, the default config for C is as below:
"code-runner.executorMap": {
"c": "gcc $fullFileName && ./a.out"
}