Autism

I tend to think world building should explain why certain races in an RPG behave the way they do. If your world building includes reasons why gnomes behave in a way where there is overlap with autistic traits then I don't see the harm.

What traits are you referring to? Are they more isolated and keep to themselves? Explain what happened to their race to encourage that behavior. Also, just because a race is isolated, doesn't mean they want that- sometimes they are forced into isolation and it's not a natural thing for them. Are they tinkerers? How did they get started in that and where do they get materials? How do they contribute to the world as a whole? If there's an option to exclude them outright, then it feels like there might be a hole in the lore of your world.

you might be overthinking a bit. I don't think there is anything wrong with having gnomes whos culture is based on some autistic traits. Like maybe it's common for them to just leave mid-conversation or they see it as an insult if you make eye contact. That sounds like a fun idea.
Just don't make autism their only trait or be afraid of not including an idea because it doesn't fit the autism stick.
As others here have said it might be more about how you do it, not if it's gnomes or any other species. The only thing I really associate with gnomes would be a low body height.

Autism can present in many ways and many "levels." Are you talking about making all of your gnomes a specific, similar presentstion (ASD 1?) Or are you talking about specific stereotypes of autism? Are you talking about giving them all your very specific brand of behaviors associated with an autism diagnosis?

Because this is all important. If you're making them all have racial traits that are like you, then I say, have at, but maybe don't call it autism specifically, because you're representing a specific type and others might feel unrepresented. Or if you're making them all have behaviors we associate with a specific type of ASD, then make sure there's room in there for them to present in their unique ways.

Example: All gnomes have ASD traits. Make a roll on a chart for all known associated behaviors, randomly assigning those traits. Maybe this gnome is bad at eye contact, but maybe that one is bad at eye contact and also hypersensitive to sound. Maybe this one is extremely interested in obscure music, or that one is really great at drawing.

There's a lot to figure out but I think you can do it! If you want to. It's cool to write things we're familiar with but also to be true to our own experience. I have ADHD but my behaviors are very different from my ADHD boyfriend, and both our types are valid.

I don’t know anything about gnomes and barely anything about TTRPGs, but this question makes me think about social vs. medical models of disability. In our modern-day society, the social model would mean that autism is only a “disorder” because society doesn’t accommodate our needs well enough (or, sometimes, at all). It isn’t a disorder to be allistic because it’s “normal”, “the default”, nothing more than that. For example, in a hypothetical society where people widely use language very literally, the behaviour of not taking things literally could be labelled a disorder. Or if the vast majority of people had sound sensitivity, then someone without it who often forgets to keep their voice down might be seen as disordered. So if you do decide to go with this idea, I think there might be something to explore there when gnomes are interacting amongst themselves vs. with other groups.

If you’re concerned about “game author compares autistic people to gnomes”, is there a different class to use that might be a better choice? (I have no idea, but it sounds like maybe gnomes have a negative stereotype?)

the more i think about it, the more lemmings in the og game seem autistic

/peanutgallery

Im assuming you already have a full testing group?

It’s hard to give a definitive answer to questions like this, as different people can get offended by things to different extents.

I do however feel making something like Autism hard-coded in a race/species is a bad idea. Why not make it more like what they tend towards? Eg: Not being autistic is to gnomes is what being autistic is to humans?

Maybe that might be better. Not too sure though. Perhaps get feedback from other people.

The exact way gnomes are presented is probably important too.

I'd say it's fine to do it your way. These things only become a problem when such coded characters are the butt of jokes for their coded-autism. Besides, there is no cultural connotations between autists and gnomes like there is, say, gays and fairies.

If, several years down the line, culture does change and it becomes offensive, just put out a new version with appropriate changes.