About GameStateSlot.cs

I want to add to SetTitleText more information

Now
SetTitleText SlotNumber date

After
SetTitleText SlotNumber Score Level GOLD HP exp

How do I get variable from GameStateMap state?

Check the CustomUI example project which has a custom GameStateSlot where SetTitleText has been overridden to include the last printed text.

https://github.com/Naninovel/CustomUIExample

Where stores Custom variables?
it does not seems GameStateMap stores Custom variables

I don't know what you mean by custom variables, if you mean the naninovel ones then they never contained such things and I don't know what you were expecting.