In errata, you wrote
Please replace current code Gameboard.H with:

class GameBoard
{
public:
// The general-purpose GameBoard allows the user to specify its dimensions
GameBoard(size_t inWidth = kDefaultWidth, size_t inHeight = kDefaultHeight);
GameBoard(const GameBoard& src); // copy constructor
virtual ~GameBoard();
GameBoard& operator=(const GameBoard& rhs); // assignment operator

void setPieceAt(size_t x, size_t y, const GamePiece* inPiece);
GamePiece* getPieceAt(size_t x, size_t y);
const GamePiece* getPieceAt(size_t x, size_t y) const;
...

But I didn't find any explanations for why you have changed the code. What was wrong with the previous approach?

In the original code, the ChessPiece was copied to a GamePiece. The information that it's really a ChessPiece was not stored in the GameBoard. Object slicing was happening: only the GamePiece part of the ChessPiece was copied to the GameBoard. To make it work correctly, GameBoard should store GamePiece* instead of GamePiece. That's what the errata is doing now.