It rapidly disperses in the sense that the gas scatters too much to actually influence anything's orbit. For you to use it to clear out any swath of space, you would need an enormous amount of gas released at a high velocity. And even then, since all the debris is constantly moving, it's not like that area is now 'clear'. You just have slightly less debris at that orbital inclination. Or slightly more debris, depending on the design of your gas bomb.