Why do you need GLM? If you ever load your vessel in a way that is not directly represented in your stability documentation, you will not know for sure that you are operating within safe limits unless you work out the weight, center of gravity and free surface numbers and know how to check those numbers against a curve of maximum allowable CG (or minimum GM) at that displacement and LCG. It takes some time to do this properly and there is always the question of whether it has been done correctly. GLM does it for you very quickly and never makes errors in arithmetic.