c++ – How can I create objects while adding them into a vector?

To answer the first part of your question, you must create an object of type Player before you can use it. When you say push_back(Player), it means “add the Player class to the vector”, not “add an object of type Player to the vector” (which is what you meant).

You can create the object on the stack like this:

Player player;
vectorOfGamers.push_back(player);    // <-- name of variable, not type

Or you can even create a temporary object inline and push that (it gets copied when it’s put in the vector):

vectorOfGamers.push_back(Player());    // <-- parentheses create a "temporary"

To answer the second part, you can create a vector of the base type, which will allow you to push back objects of any subtype; however, this won’t work as expected:

vector<Gamer> gamers;
gamers.push_back(Dealer());    // Doesn't work properly!

since when the dealer object is put into the vector, it gets copied as a Gamer object — this means only the Gamer part is copied effectively “slicing” the object. You can use pointers, however, since then only the pointer would get copied, and the object is never sliced:

vector<Gamer*> gamers;
gamers.push_back(new Dealer());    // <-- Allocate on heap with `new`, since we
                                   // want the object to persist while it's
                                   // pointed to

Question 1:

   vectorOfGamers.push_back(Player)

This is problematic because you cannot directly push a class name into a vector.
You can either push an object of class into the vector or push reference or pointer to class type into the vector. For example:

vectorOfGamers.push_back(Player(name, id)) 
  //^^assuming name and id are parameters to the vector, call Player constructor
  //^^In other words, push `instance`  of Player class into vector

Question 2:

These 3 classes derives from Gamer. Can I create vector to hold objects of Dealer, Bot and Player at the same time? How do I do that?

Yes you can. You can create a vector of pointers that points to the base class Gamer.
A good choice is to use a vector of smart_pointer, therefore, you do not need to manage pointer memory by yourself. Since the other three classes are derived from Gamer, based on polymorphism, you can assign derived class objects to base class pointers. You may find more information from this post: std::vector of objects / pointers / smart pointers to pass objects (buss error: 10)?