I've made a short video on how to use them to create a .gxt2 file:
The tutorial below is still useful if you're editing large .gxt2 files, though.
For this you will need:
*A hex editor (I'll be using Hex Workshop)
Now the important part here is that GXT2Tools only works for the PS3/360 version of .gxt2 files. The PC .gxt2 files are slightly different in that some of the information is byte flipped, so we have to unflip the bytes so the files will work in GXT2Tools.
Here is how the gxt2 files work:
Everything but the light blue 'Descriptions' in the image is what's going to need reversed. I'll refer to this image in the future.
Step #1First up, use OpenIV to extract the .gxt2 language files, they'll be under x64/data/lang/ in an rpf file. The main .gxt2 file is Grand Theft Auto V\update\update.rpf\x64\data\lang\american_rel.rpf\global.gxt2 (replace 'american' with whatever language you're working with).
Step #2Now open the .gxt2 file in your hex editor and select everything but the Descriptions (should note that the Descriptions start 4 bytes after the second "2TXG" header):
After you select the necessary block of data, you need to byte flip it (in Hex Workshop this is Tools → Operations → Byte Flip...). You need to treat data as 32 bit, and make sure you're only applying this to the selection, not the entire file. Then save the file.
Step #3Now you have a file that GXT2Tools will accept. In GXT2Tools load your .gxt2 file with File → Load GXT2, and once it's done loading (the main global.gxt2 may take a second to load) go File → Export TXT.
This will create an easily editable .txt file that you can change with any text editor. Just make sure you keep the format the same (the hash, then a tab, then the description).
After you're done editing the .txt file, open it in GXT2Tools with File → Import TXT (make sure GXT2Tools is empty before doing this, either open/close the program or hit File → New GXT2).
Save that as a .gxt2 file with File → Save GXT2.
Step #4Now all you have to do is repeat Step #2, and then import it back into the .rpf file with OpenIV.
Adding new entriesFor this I'm going to assume you have an editable .txt file (Step #3 above), and know what you're hash names are that you're adding.
If you want to add a completely new entry to a .gxt2 file you need to create a hash out of whatever name/entry you're adding to the game. OpenIV has a nice hash generator that you can use, Tools → Hash Generator.
Just drop the name/list into the Input section, make sure you're on Hex, and hit Generate.
You can add that hash directly to your .txt file (minus the 0x bit).