This would probably be better in the Gosu Exchange forum.
As far as animation goes:
1. Load the frames of the animation into an array using the tileset loader, or manually
2. Draw one element of that array per whatever time unit you will use
3. ???
4. PROFIT!
May the matz be with you.
Both Chingu and Gamebox are packages which give you a higher level interface to Gosu, which manage things like animation for you. I'd recommend using whichever one of those you like the look of. Note, however, that you should use the beta version of Chingu gem install chingu --pre
and be aware that it is no longer being developed (or use Gamebox, of course).
Good luck!