I read in the fluff that the Tau sometimes employ space marine deserters in their ranks and call them gue'vessa. However, nowhere in the codex does it mention rules for these. The Kroot and Vespid have their rules laid out, why not the gue'vessa?

Just because it's in the fluff doesn't mean it will make it into a codex.

The old codex used to have them, called something like Tau Auxiliaries or some such. They fazed them out in the newest codex.... But, if I remember correctly, if you use them as normal Tau warriors, you can model them that way and just say that they are fluff-wise gue'vessa, but game terms regular warriors (or kroot if you wanted)

It was not Space Marines...just Imperial Guard.

I was looking forward to having some real close combat capability and staying power with human deserters in my army! Oh, well, thanks anyway...