The question you're asking can't be answered. Basically when it comes to applying science to our diets there are two approaches (I think we need to use both because there are limits in either case).
First, there's the "Gary Taubes" approach. Where he cares mostly about observables. This is close to your question: I.e., "does gluten mess up healthy people"? In this situation you need to both define "mess up" and "healthy people", and then do a study and look at the outcomes at the level of "what happened to the people".
The other approach (and where I lean), I'll call the "Mat Lalonde" approach. (I probably lean this way because I'm a chemist too.) In this approach we concern ourselves with the biochemical mechanisms: Glutens cause some types of cells to "fall apart". The types of cells that it interferes with are in the gut. When the gut lining breaks down, whole proteins get into the blood. When whole proteins get into the blood you can get autoimmune responses. Etc. Here we're talking at the chemical level. And as you're concerned maybe that doesn't happen with "healthy" people and there may not be an observable.
So back to your question, you want "scientific evidence" that gluten "messes up" "healthy people". I put the quotes in there because those terms need to be defined. Lets start from the back of the question:
"Healthy People" How do you define healthy people? There are lots of people that may appear healthy but have some weird skin rash that they wouldn't attribute to gluten, they may just call it dry skin. Or someone may have a permanently runny nose, and they just say it's allergies. So you'll need some (hopefully simple and easy to defend to your audience) metric that distinguishes between health and non-healthy people
"Messes Up" What do you mean messes up? Autoimmune problems? Seemingly random diarrhea? Chronically runny nose? Skin rashes? Bloating after eating? You need to pick the metric you're comparing in your study.
"Scientific Evidence" Given that you've defined healthy people (at a level that your audience can understand) and that you've defined "messes up" to single out 1 or 2 (at most) illnesses. You need to split your people up into multiple groups, control their food, and hopefully not have any other confounds in your study.
I'm stopping here before this gets longer, but in reality science is hard, and it's even harder to get right. That's why you'll never find a "definitive" study either way that something is good or bad for you, you have to look at how the good and bad were defined in THAT study. That's why I generally lean towards the mechanistic approach, we can understand what the molecules are doing and then extrapolate and say "if I don't eat gluten, I won't damage my gut and get all the problem that come along with a damaged gut". Then, as I did, I quit eating gluten and things are better. So, FOR ME, I'm happy.
One study (Key's Seven Country Study) is the only study out there that said "fat is bad" and then people interpreted it as the definitive study that and said that fat was bad. There was no mechanistic given. Just "fat messes up people" and everyone bought it.