Gather doesn't emit light. It gathers light. It is an extra step they added to Firefly that you can invoke using node. It is poorly implemented - the way it works is that it shoots out random rays and looks at what it hits. Then it asks what it hit to compute its shader value and uses that to color the object its on, with a falloff based on distance. So it works like reflection. However, unlike reflection which shoots a ray in a well know direction every time, these rays are in pseudo random directions with a well defined probability distribution. This is called a Monte Carlo simulation. The true "gather" effect would require hundreds of thousands, or even millions, of rays before it becomes accurate (and smooth). Because no Poser user would ever tolerate a 15-day render, nobody uses enough samples to get it to look good. So it never looks good. It actually can look good, but I'm not willing to wait that long.

Other renderers speed things up with something called "irradiance caching" - which basically means cutting the world up into little voxels (3-D volume chunks, like pixels are for 2-D). As rays ask for gathered light info, the consult the enclosing voxel. The next ray that hits the same voxel does not need to calculate much since that voxel has already been calculated and cached. But making the voxels larger, you cut way down on calculations, but get smeared blurry results. Make the voxels smaller, and you get more accurate approximations, but much slower results.

Poser 7 has a render parameter called "irradiance caching". Unless they made that up, I believe they use it for ambient occlusion performance improvements. When you adjust that parameter, you're adjusting the voxel size, trading speed for quality. It may be that the Gather node uses it as well in P7, in which case you may be able to get better faster results now. You should try it. I don't have P7, and have no plans to get it. Everything I care about is either broken or just the same in P7 versus P6.