This is how it should look in the GC2Types.XML
(replacing the [ with < and ] with > of course)

[Hull Name="MediumHull4"]
[DisplayName]WhiteStar[/DisplayName]
[Description]The WhiteStar from Babylon 5[/Description]
[Cost]80[/Cost]
[Size]45[/Size]
[SensorRange]12[/SensorRange]
[HP]35[/HP]
[Model]WhiteStar[/Model]
[Thumbnail]whitestar-02a[/Thumbnail]
[Speed]19[/Speed]
[Category]Medium[/Category]
[Logistics]4[/Logistics]
[Tech_Requirement]Medium Scale Building[/Tech_Requirement]
[/Hull]

The model file itself should be renamed to S99_WhiteStar.x, which makes it available to any race that you might be playing as (the S99_ part is the race designation, and the underscore is important). And of course you would need Medium Scale Building before it would show up.

I use MediumHull4 for the Hull Name because the game already has some medium hulls, and I have added additional hulls as well. So each one gets named +1 to the previous so it shows up in the list correctly. So if this is the first medium hull you are adding it would be named MediumHull3, because the game already has MediumHull1, 2 and 3.

The instructions with all the models are very lacking in clarity I have noticed.

(And ya, I changed a few things, giving this ship some 'special' abilities, like the hit points, sensor range, and speed)

I got it to work but I have a question. Maybe Im just missing this but did you make up the values for the XML file or did you get the numbers from somewhere?

Ya, it might be a bit much. Oh well. I'm a speed freak.

darthfalz, the values were copied and pasted, then changed a bit, or a bit much as Coplann said.

Coplann, this game is a time waster for me right now. I literally have nothing much else to do. I just like to watch the space battles. And using your ships is a pleasure, they are wonderfully detailed.

And, I put them in as S99, so the AI should be able to use them too, I think. Never seen them do it though.
I did just add in a tech called Diamond Hull Design at the end of the hull techs that increases hit points by 250%.
The AI did use that one. I saw some of their ships with 300 hit points. That should make for some interesting battles, especially with the new combat model of 1.2. But, I hate to declare war on anyone. I wait to be attacked first. Never happened in that game and I ended up with a tech victory. Ah, well.

I hate to lose those nice ships, too. Speed 19 gives me running room, lol.