Good for you for thinking about this now and wanting to get ahead of the problem before it's too late. The good news is that you have a couple of months to prepare and (I assume) pretty easy access to the new dog since it belongs to your girlfriend.
As a certified trainer, I agree that this sounds like a case of resource guarding. Dogs can guard food and toys. Most people recognize that as the dog wanting his stuff to be left alone. What sometimes goes unrecognized as resource guarding is when the dog starts guarding things like the space around its human or access to high value areas like sleeping areas, food prep areas, or passage through doorways.
Right now your dog has a pretty negative emotion response to new dogs being around his stuff and it looks like he considers you and the space around you as "high-value stuff". The good thing is that he doesn't do it outside the home. That means you can start working on building a more positive association about the dobie being around when you are outside the home. That'll make it more likely that you will have success when you implement the plan inside the home.
If you want maximum results with minimum risk of things going wrong or getting worse, you need to work with a professional. Get yourself a good behaviorist or at the very least a certified trainer that is VERY experienced in integrating a new dog into a household and preventing/eliminating resource guarding in a multi-dog household. Likely your behavior modification plan will include:
--identification of and strict environmental management of resources.
--use of equipment (i.e. crates, tethers, leashes, muzzles, etc.) to physically manage the dogs when needed.
--a desensitization/counter conditioning plan for both dogs to build a positive association about them each being around the other.
--use of obedience behaviors with both dogs as a means to manage/prevent triggering the behavior in your dog.
--education for you and your girlfriend on how to recognize the subtle cues in your dogs' body language that predict or precipitate an altercation.
For best results, the plan should start now, implementing the obedience and physical management measures in the home immediately so that they are in place and practiced well before the dobie comes to live in the home. The same work should be done with the dobie in its home so that when the households are combined everyone has the same structure and expectations. Work on ds/cc protocols with the dobie can be established now outside the home and when sufficient progress has been made, those same protocols should be implemented inside the home.
If the professional you choose doesn't offer a comprehensive plan, doesn't offer support material (handouts, suggested reading, detailed homework exercises for you to follow so you can plan and track your progress) or seems to be "winging it"--WALK AWAY. If they can't explain and give detailed examples of the measures I've described above--WALK AWAY. Or if they suggest force, harsh punishment, or "dominating" your dog to eliminate the resource guarding then WALK AWAY--they are not qualified to work with you and your dog and you will not get what you need from them to ensure the smooth integration of your girlfriend's dog into your household.
Good luck.
