Writing C++ tests with "faking" some methods with an alternative implementation

Question

I'm writing tests in C++ using googleTest and want to improve them.

I try to describe what I have and what I want to achieve:

I have a class A that has an instance of class B as a member like this:

class ClassA
{
   public:
      //Some functions
   protected:
      ClassB m_b;
};
class ClassB
{
   void Init(const string &sArgument);
};

In my test I create an instance of ClassA and call a function of it. In this function the function Init() of m_b will be called.

Because Init() of ClassB makes something that I don't want to have in my test I want to call an alternate implementation. My first intention was to create an interface with the function Init() that is implemented by ClassB and by a new class that I create only for the test. Unfortunately to do this I have to create a pointer of the interface and to give it to ClassA.

__interface IB
{
   void Init(const string &sArgument);
};

class ClassB : public IB
{
   void Init(const string &sArgument) override;
};
   
class ClassA
{
   public:
      ClassA(IB* b) : m_b(b){}
     //Some functions
   protected:
      *IB m_b;
 };

For my test I create a new class:

class ClassBForTest : public IB
{
   void Init(const string &sArgument) override;
};

This is not a good solution so I tried to find another way. What I actually found is the library Isolator++ by Typemock which works quite good. I can say that for all future instances of ClassB the method Init should be replaced with another one. And the best is that I don't have to make any changes in my ClassA. I just have to create a new one:

class ClassBForTest
{
  void ISOLATOR_TESTABLE Init(const string &sArgument){}
};

In my test I use this:

ClassBForTest c;
ClassB* b = FAKE_ALL<ClassB>();
WHEN_CALLED(b->Init(ANY_VAL(string))).DoMemberFunctionInstead(&c, Init);

Unfortunately this library is not for free when I want to use it on a build server. So I'm searching for an alternative. As we used gMock in our other tests I tried to realize this with gmock but it didn't work yet. I know that this is not a good statement but I don't want code from you. I only want to know if this is possible with gMock (saying that an alternate implementation should be used without having the instance of ClassB at this point).

Alternatively I ask you for other free libraries I can use.

Thanks a lot in advance, Michaela