How can I fill the choice list of a combobox depending on a value in another cell?

You should override GetStyleRowCol and determine the choice list at runtime.

Example:

BOOL CMyGridyView::GetStyleRowCol(ROWCOL nRow, ROWCOL nCol, CGXStyle& style, GXModifyType mt, int nType)
{
   if (!base_class::GetStyleRowCol(nRow, nCol, style, mt, nType))
      return FALSE;
   if (IsPrinting() || nRow == 0 || nType == -1)
      return TRUE;
   // you may also set this in your OnInitialUpdate routine!
   m_bRefreshOnSetCurrentCell = TRUE;
 
   // Check if this is the current cell
   if (nCol == yourComboBoxCol && IsCurrentCell(nRow, nCol))
   {
      // execute a query which determines the choice only
      // for the current cell.  For inactive cells, the
      // choice list has no meaning.
      // current value of cell is available by calling style.GetValueRef()
      // values of other cells are availabe with GetValueRowCol
      style.SetChoiceList(szChoices);
   }
   return TRUE;
}