Hi there! You have gotten some great advice so far I just want to add in a handful of change (as I am often too long winded to have a post be considered 2 cents! lol)
I have dealt with dogs exactly as you describe Gus to be. My current pup, Duke (Dobermann/GSD), is 7 months was born wild and stayed wild until he was over 12 weeks of age. Zero human contact until the day I rescued him. Needless to say people are not his favorite creatures. Outside of the family and the frequent visitors he used to be fearful of people. He is currently just in the aloof stage but I have been working with him very intensely on this for the past 4 months.
My recommendations for dogs that have a tendency to plant and back when people approach is to get the dog gradually used to the body language of humans. A human that walks straight up to a dog and proceeds to lean over the dog with a hand outstretched looks to be displaying both dominant and aggressive behavior in the dogs eye - especially if the dog has ever encountered a human that uses body language purposefully in a dominant aggressive manner towards the dog. I.E. the previously mentioned daycare scenario.
We try to get dogs comfortable with the fact that humans(most of them) don't use body language the way a dog does, that some action or posture a human takes that means "AWW, come here cute doggie I wanna give you some love!!" to a human, rather than "I'm head honcho 'round these parts, submit or prepare to fight" when a dog does the same or similar posturing/use of bodylanguage.
A nervous dog will not always be nervous or react to this body language from accepted pack members. A dog can be perfectly comfortable with the dominance of its own pack and what "it's" humans body language means. A dog learns that certain behaviors from certain people mean certain things the same way a dog will observe which humans are the pushovers and which are dominant in it's own pack (in the average american dog household.)
A stranger that approaches exhibiting aggressive body language (in the dogs eye) to a dog that is already unsure of strangers, will cause various reactions from the dog depending on it's temperament. A dog that backs away with tail tucked and ears back, wide eyes, mouth shut with lips pulled back and tight, is afraid and stressed out.
Monitor the body language of Gus, when you notice him start to tense up, where his hind end seems to be getting lower and his tail is very low or curled under him, and his mouth is shut with the lips pulled back and tight, (often a dog this mervous will be looking around a lot, overly alert) but hasn't started to back away or shy away yet, make him sit or down. Draw his attention to you immediately.
I think you should bring that favorite ball of his along with you. I also agree not to flood him or attempt to sensory overload him, but I think pet stores are a great place to get a dog used to humans, and used to focusing on you instead of those humans. It is an atmosphere where dogs are common and will not always draw so much attention if it isn't wanted.
Bring that ball, keep your eyes on him. You may want to set up an exercise with him and someone you know that is a stranger to Gus so that you can monitor his body language in a controlled atmosphere first. Do it outside of the home, watch Gus's body language, the moment he starts to shut down into stress/nervous/uncertainty mode, bring out the ball, ask for his attention with the look at me game, make the "stranger" not come any closer but not retreat, and not looking at Gus in the eye.
Once Gus even glances at you as you are asking, give the ball as a reward. Try to keep his attention on you, say his name, show him the ball, put a hand on his collar. But do not praise or pet or coo to him if he is afraid and showing fear, especially not if he backs away. Also do not correct him unless he becomes aggressive. Do what you need to in order to get his focus on you. Ask him to sit, praise that sit! Praise his OB.
We need to condition him that backing away is not going to cause the person to go away, that showing fear and submission is not going to make the stranger leave him alone, but we don't want to push him. Have your friend walk parallel to you and Gus at a distance that is just barely within Gus's comfort level. Gus is likely to be quite distracted by the stranger, he may bump your leg or attempt to cross behind you, or run into you. Just keep walking at a steady pace, if this means keeping some tension on the leash for a little while or holding it at exactly the same spot to keep him the same distance from you, that is ok. He may lag-tug, then catch up quickly, and tug and look and bump you then catch up again, that is ok also. Just keep him moving forward at a medium-slow pace. Don't look at "the stranger", focus on Gus and your forward movement.
Ask for his attention, saying his name, or with use of the ball, etc to get him to look at you (the look at me game) and keep walking. Don't go too fast, but if he starts to lag a little keep walking forward, continue your forward movement to encourage him to also continue moving forward.
With the stranger walking with you, not towards, or away from you, you prevent Gus from feeling as if he is retreating, and also he is not being forced to advance on this stranger. He is not being submissive and fearful, or dominant and aggressive.
Have the stranger slowly get closer once Gus has been regularly obeying the look at me.
This, I find, is always a good place to start with shy or fearful dogs that have not displayed aggression. I never force a dog to advance or accept a scary thing, but I also never allow the dog to run away or retreat. When Gus becomes uncomfortable, put him in a sit-stay or a down-stay, ask for his attention, and do not allow him to back up to the end of the leash. If he does back up, have the person stop and not come any closer, and call him to your side, focus on him, block the other person with your body a little so that Gus knows you want him to advance towards YOU, you are not asking him to advance/approach the stranger.
No cooing or soft talking that could be taken as praise, but do not be harsh, just calm and firm.
Without seeing him myself it is hard to say how he will react to all this, how hard it will be to get his attention back on you, etc. But hopefully worth a shot for you. I wish you luck, please keep us posted on what you try and how Gus reacts!
~Jenn