In order to replace the default auto-generated column with GridViewComboBoxColumn, you should subscribe for the AutoGeneratingColumn event. Once it is raised for the column, you can replace e.Column with a new one. In case you prefer to not create a new column, you can also directly define a CellTemplate for the already generated column (e.Column).
You can learn more about how to configure GridViewComboBoxColumn properly in our online documentation here: ComboBox Column.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.