The LBGR (Lower Bound of the Grey Region) is a concentration. It is less than the DCGLW and is chosen to be easily distinguishable from the DCGLW. You apply a statistical test to the data to determine whether the true concentration in the survey unit is above the DCGLW or below the LBGR. You are more likely to make an inaccurate decision if the true concentration of the survey unit is between the LBGR and the DCGLW. Thus, it is called the "grey region" because in that case, the decision is usually neither "black" nor "white."
See, Chapter 5: Survey Planning and Design and Appendix D: The Planning Phase of the Data Life Cycle on the MARSSIM Manual download page.