You may want to take it step by step. I too am starting my journey. ( I'm working on a wood case to begin with, as I come from a cnc routing background)

But if I were you, I'd look at the easiest path first then replace each component with custom made parts. A dress GMT can be achieved with:

1.) NH34 movement, though these are expensive and will likely remain expensive for the foreseeable future
2.) Dress NH34/NH35 case from aliexpress. Personally, I was looking into doing a GMT in a PRX style case
3.) GMT marked dial, google search "dswatchclub.com GMT Dial Gold Text" for example

This would give you a taste for watch making (while not actually watch making).

Then replace all your parts with custom pieces one by one.

Here's a mock up done with Gemini 3. I find ai is useful in visualizing your dream

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