First you will have to enlarge the existing partition to use the full disk space. I don't know if you can do it with the GUI you show. From the command line you can use fdisk. ~This seems to be old-style MBR partitioning, so that should not be a problem~.
No I was wrong about that, the simple disktool in the screenshot mistook a MBR compatible GPT partitioning for MBR partitioning.
To check if a disk uses GPT, you can use
gdisk -l /dev/sda
If it uses MBR it will warn:
Found invalid GPT and valid MBR. If the disk uses GPT with MBR compatibility, it will say
Found valid GPT with hybrid MBR; using GPT. Otherwise it is plain GPT with no MBR compatibility.
For partitioning GPT disks, you have to use gdisk. Also you have to use its expert mode and relocate backup tables to end of disk, otherwise you can't enlarge a partition.
This should be the starting partition table you had.
Number Start (sector) End (sector) Size Code Name
1 227328 62916574 29.9 GiB 8300
14 2048 10239 4.0 MiB EF02
15 10240 227327 106.0 MiB EF00
You can use gdisk then to edit number 1:
e (relocate backup)
n(ew) start sector as original, set end to end of disk
w(rite) and exit
Maybe you have to reread the partition. Usually this will autoupdate. Otherwise echo 1 > /sys/block/sda/device/rescan. Executing lsblk should show the right sizes.
Secondly you have to expand the filesystem itself. You show here ext4, so this can be done online with resize2fs /dev/sda1