It may be a good idea to communicate with your gallade—specifically through means your new traveling companion can’t eavesdrop on. Gallade are highly protective pokémon, but it’s also important to remember that in addition to this protectiveness, they’re also psychic. While it’s entirely possible that your gallade is simply being overly cautious around your friend, it’s also possible that your companion presents a very real danger that your gallade has picked up on. Communicating with your gallade through subtle channels will help you determine whether or not the threat is valid and what actions you would need to take after that.
If your new companion is indeed a threat (and, of course, always investigate before assuming your gallade is completely correct), it’s important to take caution in your dealings with him from that point forward. I would advise parting ways with him in a public area (and doing so politely but confidently) to minimize the possibility that he will respond violently. Be sure to have your pokémon close at hand for a few days afterwards for protection, and try not to make it obvious that it’s because you perceive him as a danger to you.
If, however, your companion is not a threat, try to explain to your gallade that your friend is trustworthy. I would also suggestion having your friend and your pokémon engage in team building exercises (such as, for example, allowing your friend to help take care of gallade or having your gallade and your friend work together to set up camp) so that the two of them may begin to build bonds with one another. Eventually, the longer your friend works with your gallade, the more likely your gallade will finally let his guard down.
Good luck, anonymous!
The Pokédex According to Bill 