I don't have experience with CFCs (I use a whirlpool IC), but would think that you could still do it via gravity, except that it will be slow (as with any gravity-fed CFC). The risk with this is that the time it takes to chill all of the brew will be fairly long, causing a good portion of the wort to sit around hot for an extended period. You then risk increased DMS (because the DMS pre-cursors had a chance to convert in the hot wort), decreased hop aroma (which can be overcome by dry hopping) and increased cold break into the fermenter. I think the latter will also increase your chances of chill haze in your final product. As you likely know, the goal of a chiller is to get the wort under 140F as quickly as possible. I target getting under 100F in 6 mins or less. So, you might consider getting a pump.
Folks who have CFCs should chime in since my thoughts on this are purely academic.