@msstice
Gump is an interesting type case.
I'm not a fan of the movie, I didn't like it at the time, and I think it has aged even more poorly. But I also think it was a rare experiment from Hollywood and interesting failures are far more important IMO than formula successes. It is deserving of our attention.
I think one reason it works for many is because he clearly does have flaws, ie his handicap. That essential underdog feeling you describe. And unlike most stories, his flaws are ones he can do nothing about, except the one thing he does do---have a positive worldview. The starting point of a character like this is going to be well-received in the hearts of every audience member in possession of an actual heart.
Those unchanging flaws are then sort of held out as a white space, or a backdrop, so that we can see the effect of that positive attitude... which also needs to be static and persistent, because the change is placed in what he does and the effect he has on the world. So I consider him to be an archetype. An agent of change rather than a changing agent. And he might well be the only archetype of his sort.
Problem is that conceit---the static, archetypal, good-attitude, not-as-blessed, soldiering-on hero demonstrating change around him---strips the drama from the movie. That's why it desperately needs those life and death scenes. Nam was key to telling the story, as was Jenny dying.
I find it really sappy and unrealistic and the message delivered with a bludgeon (Hollywood's preferred intellectual weapon). It's intended as inspirational myth-making, so realism doesn't matter much, I admit. Still, the physics jar me. I do think it's decent if taken as a kid's movie. It's not a movie that suffers thinking deeply about it's story and message well.
But I do think it is a movie that suffers structural analysis well.
One such question is, would it have worked better if he had to overcome serious challenges along the way? But how would you navigate him through those challenges? What would he overcome in himself? Would it be fair to position the audience to criticize his decision-making? And that's all part of the problem. They never made him a real person in the first place, struggling with the imperfections we all have within us, handicapped or not.. But do that and now you're distracting away from theme of overcoming things out of your control AND you'll look like your beating up on your defenseless main character and asking the audience to come with. So the creative team was addressing a writing dilemma. What we see was their solution.
And that was a tough call for the writer-director-producer, I'm sure. I'm glad they tried. Every project like this one helps light the way forward for those coming after, whether as cautionary tale or roaring success.