The wxGBSizerItem class is used by the wxGridBagSizer for tracking the items in the sizer. It adds grid position and spanning information to the normal wxSizerItem by adding wxGBPosition (not implemented in wx) and wxGBSpan (not implemented in wx) attributes. Most of the time you will not need to use a wxGBSizerItem directly in your code, but there are a couple of cases where it is handy.
This class is derived (and can use functions) from: wxSizerItem
wxWidgets docs: wxGBSizerItem