The gathering foot is a little foot about the size of the clear small foot that came with the machine. You set your tension tighter and adjust the stitch length. It makes loose gathers (O.K. for small things) I think I paid around $10.(I had it for my 6600 and it fits my 7700) It is for sale at Brubaker's on line for $12.99 (no affiliation)
The ruffler is a big clunky looking attachment, but you can control the gathers. Good if you are making lots of ruffles. The Janome part # is 846415008 for the accufeed one.
I found that one at Brubaker's site also and it is $96.
There is a video on the Janome site about the accufeed one.
I just have the gathering foot.
I hope this helps. Elizabeth