Re-routing a real function to a mocking function with Gmock?

When I researched Gmock from Google. I've installed and built the project work well so far. But I have some concern about mocking function. Now I have some files:

<1> myGtest.h

#ifndef MYGTEST_H_
#define MYGTEST_H_

int test(int);
int function(int);

#endif /* MYGTEST_H_ */

<2> src_code.cpp

#include <stdio.h>
#include "myGtest.h"

int test(int a) {
    printf("NOT overridden!\n");
    return a;
}

int function(int a){

    int x = test(a);
    if(x == 0)
        return 99;
    else
        return 0;
}

<3> myGtest_dummy.h

#ifndef MYGTEST_DUMMY_H_
#define MYGTEST_DUMMY_H_

#include "gmock/gmock.h"
#include "../myGtest/myGtest.h"

class myGtestMock
{
public:
    myGtestMock(){};
    ~myGtestMock(){};
    MOCK_METHOD1(test, int(int));
};

#endif /* MYGTEST_DUMMY_H_ */

<4> test_program.cpp

#include <gmock/gmock.h>
#include <gtest/gtest.h>
#include "src/myGtest/myGtest.h"
#include "src/dummy/myGtest_dummy.h"

using testing::_;
using testing::Return;
using testing::InSequence;
using ::testing::AtLeast;

extern int function(int a);
extern int test(int a);

class BTest:public testing::Test{
public:
    myGtestMock mock_test;


    int __wrap_test(int a);
};

int BTest::__wrap_test(int a){

    printf("overridden!\n");
    return a;
}

TEST_F(BTest, CallMockTest) {

    EXPECT_CALL(mock_test, test(0))
            .WillOnce(Invoke(this, &BTest::__wrap_test));

    function(99);
}

int main(int argc, char *argv[]) {

    ::testing::InitGoogleTest(&argc, argv);
    return RUN_ALL_TESTS();
}

Could you help me explain that: How can I mock function "int test(int)" ? I'd like to expect that once "TEST_F(BTest, CallMockTest)" is executed , the program call "function(99);" THEN: my mocking function "int __wrap_test(int)" will be called instead of "int test(int)".

Thanks so much for the answer.