If you need to ask these questions then you won't do it yourself.
This means it's cheaper to sell the transmission and buy a gasoline one, as this is a big job and not every tuner is going to manage to do it right either.
The tune is gonna cost more than a gasoline trans.

You are better off finding a petrol box which is already calibrated close to how you want and focusing on making the CAN messaging and torque reduction requests work properly on whatever "custom made ECU" you have (this is a big task in and of itself, I hope you have a good understanding of torque management!) rather than trying to do two things at once.

What prj is saying is that if you can't figure this out using Google, you're going to really struggle with actually doing the work.

Virtually any aftermarket tuner (PCM, bFlash, Autotuner, MMS, KESS, Trasdata, whatever other stuff has crawled out of a swamp lately) will flash DQ250.

You should probably understand how control modules work before you undertake this venture. Any hex editor can edit the calibration area for a control module, it's just data. WinOLS is the best enhanced hex editor because it has good tools for comparing files and finding map data by looking at patterns. You will want map definitions (DAMOS/A2L, OLS map pack, etc.). If one isn't available for your DSG software version, you will need the ability to cross-correlate maps by hand by looking at similar files and patterns in the data. WinOLS has good tools for this but it's a skill.

As for the diesel gearbox: I don't know Skoda Superb particularly, but my guess is that this diesel box was cheap for a reason. The gear ratios are going to be fundamentally bad for petrol and you will need to do extensive modification of the DSG calibration. This means you will need a definition file with very complete map definitions (ideally an A2L) which may be hard to find for your gearbox software version. You might not be able to see the maps you need to edit with the common bad OLS projects out there. And you will need to learn the full control strategy of the gearbox (Funktionsrahmen will help here) which is time consuming.

If you write a petrol file it won't go into gear and/or destroy the synchros, because the gearbox ratios are wrong.
If you correct the gear ratios it will shift up and down non-stop in a loop because all the shift maps are wrong.

This is a very difficult task for those who do this for a living, you can't even use google.
Buy a gasoline box and forget it. Even if someone makes it work the gear ratios will suck anyway for a gasoline engine.

Your mechanical engineering background will clearly not be of any use, because it is probably in a completely unrelated field.

Asking this question shows complete lack of understanding of how the gearbox works.

The questions you are asking are not technical. They are something that you can find in <5 minutes of Google usage.
If you can't accomplish that, you are never going to get this gearbox to work, end of story.

You're not entitled to any help. But you are receiving a recommendation from someone who did this for a living for over ten years.
I've done the exact thing you're trying to do. It took me over 20 hours the first time, I knew exactly what I was doing.
Re-calibrating all the shiftmaps from complete scratch and sorting out the shift gradients just takes a shitton of time and driving, and that's when you have all the tools and data + full knowledge of how it works.

You're gonna take 200+ hours. I don't care if you got the box for free, it's cheaper and much better for the end result to buy a petrol box.
Playing victim here and whining is not going to get you anywhere whatsoever, nor is anyone going to give you a handout.

Great, come back here when you have a real calibration related question. Until then, goodbye.

I disagree, they're not close, especially in the first few gears.
All the shift maps and gradients still have to be redone.
Also 3.6 is not a good file to start with, because the stall speeds and a bunch of other stuff is not suitable for a turbocharged 4 cylinder...

Connect with VCDS and check the "Advanced ID" tab for mech unit code. Also there is a software identification code somewhere in the measuring blocks.