[G2 Q&A, Help & Troubleshooting] Kickoff:
Revised quide

Ok, I'll try to provide a more comprehensive explanation. I assume that you have a fully working phone, the hardware is 32 GB but you can use only 16 GB (10,7 GB). The files we will use are

https://www.dropbox.com/sh/s19w3acg2i4y1jh/AADlrbamYu51KZysutnYj9FMa?dl=0.

Let's start with downloading the necessary files to your SDCard.

You have to be on KitKat, this solution does not work on Lollipop, so downgrade first if necessary.

On the phone use Total Commander to copy the files to /system/xbin and change the permission to 755 on each. Then reboot the phone.

Then type via adb:

adb shell
su
cd /system
parted /dev/block/mmcblk0

Many users reported not to get the following Error messages here:

Error: The backup GPT table is not at the end of the disk, as it should be.

This might mean that another operating system believes the disk is smaller.

Fix, by moving the backup to the end (and removing the old backup)?

Fix/Ignore/Cancel? Fix

Warning: Not all of the space available to /dev/block/mmcblk0 appears to be

used, you can fix the GPT to use all of the space (an extra 30294016 blocks) or

continue with the current setting?

Fix/Ignore?Fix

No problem, check your partitions with "print"

1|root@android:/ # parted /dev/block/mmcblk0
parted /dev/block/mmcblk0
GNU Parted 1.8.8.1.179-aef3
Using /dev/block/mmcblk0
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print
print
print
Model: MMC 032G96 (sd/mmc)
Disk /dev/block/mmcblk0: 31.3GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number Start End Size File system Name Flags
1 16.8MB 83.9MB 67.1MB fat16 modem
2 83.9MB 84.9MB 1049kB sbl1
3 84.9MB 85.5MB 524kB dbi
4 101MB 101MB 524kB DDR
5 117MB 118MB 1049kB aboot
6 118MB 120MB 1049kB rpm
7 134MB 151MB 16.8MB boot
8 151MB 152MB 1049kB tz
9 152MB 152MB 1024B pad
10 168MB 171MB 3146kB modemst1
11 171MB 174MB 3146kB modemst2
12 174MB 174MB 1024B pad1
13 185MB 201MB 16.8MB misc
14 201MB 235MB 33.6MB ext4 persist
15 235MB 252MB 16.8MB recovery
16 252MB 255MB 3146kB fsg
17 268MB 269MB 524kB fsc
18 269MB 269MB 524kB ssd
19 269MB 269MB 1024B pad2
20 269MB 270MB 524kB encrypt
21 285MB 294MB 8389kB ext4 drm
22 294MB 302MB 8389kB ext4 sns
23 302MB 336MB 33.6MB laf
24 336MB 369MB 33.6MB fota
25 369MB 403MB 33.6MB ext4 mpt
26 403MB 403MB 524kB dbibak
27 403MB 404MB 1049kB rpmbak
28 404MB 405MB 1049kB tzbak
29 405MB 405MB 8192B rct
30 419MB 3322MB 2902MB ext4 system
31 3322MB 3959MB 638MB ext4 cache
32 3959MB 4043MB 83.9MB tombstones
33 4043MB 4060MB 16.8MB spare
34 4060MB 4110MB 50.3MB ext4 cust
35 4110MB 31.2GB 27.1GB ext4 userdata
36 31.2GB 31.3GB 62.9MB grow

This is what I see if the phone is OK. In your case line 36 should look like:

36 15.7GB 15.8GB 37.7MB grow

Quit parted with

quit

And from here follow the original description:

adb shell
su
cd /system
dd if=/system/bin/sgpt32g.img of=/dev/block/mmcblk0 bs=512 seek=61071327 conv=notrunc
dd if=/system/bin/pgpt32g.img of=/dev/block/mmcblk0 bs=512 seek=0 conv=notrunc

After finished manually turn off the phone and use the key combination to enter Download Mode. Flash the tight KDZ file or make a recovery via LGMobileSupportTool.

After the first boot you will see that the SDCARD has only 10,7GB only. Make a hard reset Settings/Backup and reset/Factory data reset.

After resetting the phone you will have a 25GB SD Card.