Dear Radhika, By attending GB training, you learn the basic statistical tools used in Six Sigma, and this is very essential, for understanding the statistical tools taught in BB. Thus undergoing a GB before BB, helps you to understand the tools in BB more easily. Now for your question, this organization allows a GB certified only, to take up BB.
I agree with Mr. Suresh Jayaram. There are some institutions, which specifies, the financial benefit or the savings, unless not reached, they do not recognize the project. I would say, that any project, which brings in a good change in the process or considerable benefit to the employer, is good enough.