Is it possible to show group column headers in the same line?

This feature is available out-of-the-box starting from version 15.2.4. Use the GridView.OptionsView.ShowGroupPanelColumnsAsSingleRow property. 

For earlier versions:


This example demonstrates how to display group column headers in the same line, so, the group panel's height is as minimal as possible.
  
To accomplish this task, it's necessary to create a custom grid as described in the How to create a GridView descendant class and register it for design-time use KB article. Furthermore, inherit the GridViewInfo and override its CalcGroupPanelRowDrawInfo and CalcGroupPanelHeight methods.