Here's what the documentation says about layout in a GridItem
:
The GridItem container defines a grid cell in GridRow container. (The GridRow container, in turn, defines a row in a Grid container.) The GridItem container can contain any number of children, which are laid out as in an HBox container. If you do not want HBox layout, create a container, such as a VBox container, as a child of the GridItem control and put other components in this child container.
So you might try:
<mx:GridItem>
<mx:VBox>
<mx:Text text="blah blah" visible="some condition"/>
<mx:Repeater dataprovider="something something" visible="xyz">
<AnotherFileCalled paddingLeft = "5">
</mx:Repeater>
</mx:VBox>
</mx:GridItem>
or
<mx:GridItem>
<mx:Text text="blah blah" visible="some condition"/>
<mx:VBox>
<mx:Repeater dataprovider="something something" visible="xyz">
<AnotherFileCalled paddingLeft = "5">
</mx:Repeater>
</mx:VBox>
</mx:GridItem>