Hi there, I'm Matt Sergei (do call me Matt, please) and can create your PowerBI custom visual -
given enough time as in my bid. Yes, as one of the top D3.js developers here (have to fix incomplete D3.js work by others on Freelancer at least twice per year) the Gantt chart PowerBI custom visual would be implemented with D3.js and of course relevant HTML and CSS code.
Now, I'm not experienced in creating these custom visuals yet have read all Microsoft documentation how to create them. I'm using MacBook Pro exclusively so would develop your control on Mac (Node.js is used for controls creation, supported by Microsoft) and will check the output in Windows 10 as well.
You would need to specify exact requirements for your control which I think you have done quite well. For sure I don't like major changes when the draft control is made, that's all.
Looking forward to your reply - let's discuss first - and hope you're safe,
Matt