If your section is in a flow action,then you can run a pre-activity and do obj-sort on the pagelist which u are using in Repeating Grid.If you are using Report Definition You can do it directly in the report definition itself.Make sure to uncheck the enable sorting option on the grid operations.
I am populating grid by inserting a new page into a pagelist via data tranform on click of a button, and no flow action is used, and no report definition is involved as well. Please advise further in this scenario.
Grid layout basically displays what ever input the data source feeds it, so if you want to display a list of rows sorted in descending order, then you can prepare data that way and feed it. Hence forth Grid will use that data to operate on.
If you have a report definition in place then you can perform the things suggested by @Mounika. If it is a embedded pagelist property then I would recommend you to use "Sort" action in the Data Transform rule form instead of using acivity with Obj-Sort method.