Hi Rana,
Yes, the TabItem is a content control and it can have only one direct child as content. You can always wrap the GridView in a panel and add the other controls there:
<telerik:RadTabControl>
<telerik:RadtabItem>
<Grid>
<GridView></GridView>
<AnotherControl></AnotherControl>
</Grid>
</telerik:RadtabItem>
</telerik:RadTabControl>
Greetings,
Miroslav
the Telerik team