This examples creates a new empty Gui sensible to Drag n' Drop event:
Gui, Dropper: New
Gui, Dropper: Font, s10 w700
Gui, Dropper: Add, Text, y80 vText1, Drag the files here
Gui, Dropper: Show, w200 h200 Center, Dropper
return
DropperGuiDropFiles:
DroppedFile:=A_GuiEvent
FileRead, content, %DroppedFile%
msgbox %content%
return