We build this ourselves. It's React with little in the way of 3rd-party "magic".

I also needed something similar but instead of a lack of libraries, I actually remember seeing an overwhelming amount of choices. There are 200+ libraries with the word gantt in it: https://www.npmjs.com/search?q=gantt&ranking=popularity. And a lot more if you also include calendar timelines like https://github.com/namespace-ee/react-calendar-timeline.

Did you evaluate them and consider all of them to be bad? I ended up building my own because everything looked too flexible for my use case. I only needed basic layout/time tick marks and zoom/pan.

You're right, there are no good and free JS libraries for gantt charts (or even timelines). It is a non-trivial problem domain.

I know, because I helped build a product (https://peachyplan.com) much like Notion's timeline view :)

The notion team did a phenomenal job. This is one of the best timeline views I've seen, and I've looked at hundreds.

we use https://www.bryntum.com/

Obviously they built it, it's really not hard

We use smartsheets. It has Gantt chart