What you need:
X GXT Editor - This one needs administrator privileges and will also automatically update from the web. Sort of annoying but it does the import/export job well so I recommend it.
Notepad++
Pick whatever language .gxt file that you want to edit from main folder>common>text>*.gxt
Remember to make backups
Open it up with X GXT Editor.
Ctrl + A to select all.
Right click > Export to a folder
In that folder that you have all your exported files, Ctrl+A to select all
Right click > Open with Notepad++
It'll take a short while to open all the files
Ctrl+F to to open the find and replace dialogue.
You'll notice Find in all files and Replace in all Files.
Make sure to check Match case and word exactly.
Start with full names. So for example, you want to change Nico Bellic to Johnny Klebitz. Search for Nico Bellic and replace with Johnny Klebitz. Then replace any Mr. Bellic > Mr. Klebitz (this really only matters if you intend to modify the Mr. to Ms. or something similar.
Then you can start replacing the individual names like Nico to Johnny or Bellic to Klebitz. Look on GTA Wiki for a list of nicknames that each individual character has. Niko has NB, Nicky, while Johnny has John boy, Johnny boy, JK.
