I don't think you need to use any sort of formula for this. This sounds more like you just need to ad a project parameter for sheets like for instance "sheet_order" to the project. You can then assign numbers to all the sheets and order them in the schedule by these numbers. Lastly you can right click the "sheet_order" column and hide it. There might be other solutions but this one is quite simple.

also: "...but the first thing I'd do if yo'r copying Sander's formula is get rid of all the spaces and especially the slashes from the parameter names."

True, they were just examples. I prefer using underscores as the only non letter or number characters myself too.

Thanks.This is what I did at the moment.Create a new parameter using value of number and sort them out and its working but need to set them every sheet.

We use a "hundred series" for our Sort Order parameter.
In other words, all of our Front End sheets (your "G" series) uses 100.
All of our Civil and Landscape (I'm guessing your "C" and "L" uses 200.
Structural uses 300, Architectural uses 400, etc.
Then the Sheet Index sorts first by Sort Order and then by Sheet Number.
Uses the "hundred series", every "G" sheet gets 100, every "A" sheet gets 400.
Then the Sheet Numbering kicks in and sorts A-001 before A-002.
You don't need to be more sophisticated than that unless an individual discipline doesn't name their Sheets in order.
If they use some screwy scheme, THEN you can go to the "tens series" (400, 410, 420, 430) or even get super-specific and go to a "ones series" (400, 411, 412, 413, 420, 430)
But if, once you get into a discipline, those sheets are alphabetical, you can just assign everything in that discipline to the "hundred"

I do something real simple, I add a dash in front.

No one has ever mentioned it, commented or complained about it. Even if perhaps it's "not to standards".