You would not use gels or filters. In post you would turn the color to b&w. On full-up cameras, you can turn off the color, but not on most consumer or prosumer cameras. If you are shooting film, you would have to use b&w stock, or else, again, shoot in color and turn the color to b&w in post.
Note there was a post about this topic recently, and one person wisely pointed out how if you put two colors together that have the same b-w intensity, they will look the same and not separate. One way to check for this is to use an external monitor and turn off the color entirely.