According to
this post, .gds files are just gzip files with a 256 byte header. If that's the case, you should be able to just remove the header (either with a hex editor, or a program like
File Trimmer), rename the file to .gz, and extract it with the appropriate program.
To convert a save to .gds, you would gzip the raw save, paste a .gds header onto the front (either with a hex editor or with the Copy command from a command prompt), and rename the file to .gds. You might need to use a header from a save for the same game, or you might be able to use a header from any game.
These are all just guesses, as I don't have a gameshark DS to test it with, but that's what I would try.