European here: 900m makes sense (and I see it a lot, 900 m (space) doesn't. I tried '400 m run' in youtube and all results came back as '400m'.

I often see things like 500hz or 500g without the space. Are these incorrect too?

The safest answer is to consult your preferred publisher's style guide; there may be differences of opinion on issues like this.

https://physics.nist.gov/cuu/Units/checklist.html, for example, says (item #15):

> There is a space between the numerical value and unit symbol ...

(although they fail to recommend making it a non-breaking space, which I think would be wiser).

There's plenty of further discussion to be found; e.g. https://forum.wordreference.com/threads/typography-rules-for...

By the way, it's Hz not hz