Charles Webster , Jan 04, 2009; 12:27 p.m.
A barn door is not a diffusion device, it merely provides a means to keep the light off areas at the sides of the strobe coverage.
Diffusion gel will not make your light source any larger than the diameter of the gel itself (typically 7"), your shoot through umbrella will make your light source as large as the umbrella (typically 36-48"). They are not interchangable for your purposes.
For portraiture, you typically want a large light source close to the subject, so the light "wraps around" the subject, making soft light-shadow transitions. Diffusion gel won't help you achieve this.
I'd suggest sticking with the shoot through umbrella, or better yet a softbox.