DSCompress is a GZip compression front end for the Nintendo DS. It uses the latest libfat, so it should be compatible with any homebrew compatible flashcard.

Instructions
Select a file in the browser, and press A to compress.
Select a file ending in .gz in the browser, and press A to decompress.
Select a directory in the browser, and press A to enter it.
Select ‘..’ in the browser, or press B, to go up a directory.

http://voidptr.org/phpbb/viewtopic.php?t=26&start=15