Selecting all items on GM Layer at once

I have a big map of Port Nyanzaru that I would like to copy and duplicate for a certain event.  I can copy and paste the map easy enough.  But I am having a hard time with the items on the GM Layer.  Is there a way to select all items on the GM layer all at once and paste them on the duplicate map?  Someone said Ctrl-A but that doesn't seem to be working for me.  Thanks.

Within the same game?  Can’t you just draw a big square around it all then right click and copy / paste?  I do that with the players token all the time.

You cannot copy and paste between games if that is what you are trying to do. You could use the Transmogrifier to do this between games you own, of you had a Pro subscription. There is no easy way to transfer items to someone else's game, though.