I've been wondering about this myself so hopefully someone will come along and show us which button we click to make that happen

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What I've been doing is laying out a grid of 1024x1024px color fill layers overtop of my GP texture source. Each color fill layer is named according to it's corresponding texture sheet, I then Ctrl+Click on each layer's icon to load it's selection, then do copy merged (Ctrl+Shift+C) and paste that onto my texture sheet (the color fill layers are hidden of course when I do this).
I had to do a Save As for each individual tile initially, but as I edit and update my GP textures I recorded an "action" to export tiles which makes it go pretty fast. I open my folder full of tile sources (GP_01.psd, GP_02.psd, etc.) and split the screen so I can drag each one into PS without tabbing through windows.
I'll load the selection for GP_01 for example, then drag GP_01.psd into PhotoShop and click the button to run the action. Done. The action switches documents, does copy merged on the source, switches documents again, pastes the copy, merges that into the existing layer, then saves and closes GP_01.psd.
So then I load the selection for GP_02, drag GP_02.psd, run the action again and so on. It happens quickly and after you've done it a few times you can walk through 100 or more tiles in just a few minutes.
Once I've updated all the .psd tiles I use an Imagetool shortcut in my right click > Send to menu to process them all through Imagetool, converting them to .dds for the texture folder, and then another Send to option to send the converted textures to my Texture folder.
Jim