I would rather use:

int countGC(char * _gcString) {
        string gcString(_gcString);
        int res(0);
        for (int i = 0; i < gcString.length(); i++) { if (gcString[i] 
+== 'C' || gcString[i] == 'G') { res++; } }
        return res;
}
[download]

Cheers :)