If I would sow the seed, I would just scatter the seeds in a confined but well prepared area. But then when the seedlings are about 1-2" tall, I transplant and spacing them about 6" apart. My point is that you space the seedlings not the seeds. I start my Gai-lan seedling inside now in my zone.

On my package, it said to space it between 8"x16" apart. I've spaced it maybe 6" apart and it did well. You can harvest the plant several times in a season. Cut the stem just above the 1st leaf and new shoot will develop above the leaf for second harvest.