So I know that it's supposed to be used for cards from the same set, but I would like to know: Is there a way to import cards all at once from different sets with their set specified? So like a Gatecrash Stomping Grounds would be listed as "Stomping Grounds [GTC]" or something along those lines?[/GTC]

Yes, there is. You can import cards from a CSV (comma separated values) file, including set info or any other info. The easiest way to see how the file should be formatted is to export your inventory first - this will give you a CSV file (openable by Excel) with the correct headers. Simply replace the card info with whatever you want to import, then select "Add Cards" > "From a CSV file" on Deckbox and choose that file. You may need to use 'Gatecrash' instead of '[GTC]', but you can easily use the 'find and replace' feature of Word or Excel to do that. That same feature can alter other formatting differences to match Deckbox's too.[/GTC]