To mod backpack capacity with Gibbed can be a long process. Mainly due to it not increasing the same way as the ammo pools do. What you are going to have to do is basically trick the game. I'm not going to tell you how to use Gibbed, where to get it, or explain any other aspects of it here. This is just how to increase backpack capacity using Gibbed.
Under Raw, select BlackMarketUpgrades.
The first seven are your ammo. The last two are backpack & bank.
So if you have bought all upgrades from Earl(& haven't modded any yet)
it'll read:
5
5
5
5
5
5
5
5
5
Now you want to change the last two back to 0. Return to your game & buy the backpack SDUs from Earl again. Now you will have 40something spaces.
Repeat the process as many times as you want.
This will take some time to get a large backpack.
If you want to do it quicker use this http://www.xpgamesaves.com/topic/518...99#entry235899
Credit to Dack over at XPGameSaves.com.