Can I thaw whole Ginger root to use later?
No, you can never restore a fruit or vegetable to its original texture after thawing. Physically, this is caused by the water in its cells expanding and bursting the cell walls. When it thaws, the water flows out of the damaged cell walls. It is not actually pure water, but cytoplasm, so it can feel differently from plant to plant, in this case slimy. It is not always slimy, but any thawed plant becomes soft and mushy, and there is no way around it. 

While it may superficially look like rotten, there is no safety problem with it. You can use and eat it without any problems. The advice you found for using frozen ginger shows that, at least for the recipes it is intended to, the change in texture doesn't matter. If it matters for your recipe, then no, you cannot preserve it by freezing, because there is no way to restore it to its fresh state.