I'd be careful using wholly-owned subsidiary. A subsidiary is usually rendered as "Tochtergesellschaft" in German and saying that it's "wholly owned" means that its parent company owns all of its shares. For example, Ford Credit is a wholly-owned subsidiary of Ford Motor Co., which is its holding company. 100%-Gesellschafter means that someone or something is itself the full owner. Because it made headlines that this GmbH (or LLC) was sold to an American company, you could think of it as meaning that XXXX BAU GmbH previously controlled all of its own membership interest, but now sold all or part of that interest to the outsider. (Alternatively, if this can be confirmed by context, it might mean that XXXX BAU GmbH wholly-owned another company that's been mentioned and now it, the owner and parent company, has been sold off.) If you log onto http://www.roeschnet.com/histor.htm, you'll notice to the right that the authors use the term 100%-Gesellschafter as a synonym for the company that owns all of the shares or membership interest in another company.
Good luck.