Gaara specs - Gaara would have the full knowledge and the techniques that he had before being captured by Deidara and for the sake of the argument, Gaara has access to tailed beast transformation and the friendship of the first tails (being it is obvious that he would not be able to handle Killer B Without this).
Killer B Specs - Full specs and knowledge from the great from the fourth great ninja war and being he had samehada that is included.
The terrain for this battle is going to be in a plain making it fair for both 'contenders', this plain is going to be identical to the plain where the Kimimaro vs. lee took place.[Only photo I could find with a clear idea of the image I was trying to make.]