Haven't worked with UWP before, but looking at the docs, it seems like the drag and drop functionality is not supported by the regular GridView. In this case, you may need to build a custom control by extending the GridView control just like in this example:
Extending GridView with Drag and Drop for Grouping and Variable Sized Items[
^]
Looking back at the
documentation, To do multiple selection, you have to use SHIFT or CTRL keys. Since a UWP app can target different devices including touch screens, they've designed it that way.
Also, I'm not sure if this will work for you:
https://stackoverflow.com/questions/35890625/win-rt-xaml-gridview-drag-select-multiple-items