Mount encrypted volume in Debian

I was given a HDD that was encrypted using dm-crypt and I'd like to mount it as /disk2 preferably having the decryption password stored in a file, so I won't have to enter the passphrase when booting, but it's not that important.

When I try to open the disk in the file manager providing the encryption passsword, I get this error

Failed to mount "500 GB LVM2 Physical Volume".
Not a mountable file system.

lvdisplay gives

LV Path /dev/disk2/disk2
LV Name disk2
VG Name disk2
LV Status              NOT available

ls /dev/mapper gives the following, while the desired result should be disk2-disk2 I guess

udisks-luks-uuid-.....-uid1000

dmsetup ls --tree returns

udisks-luks-uuid-.....-uid1000 (253:7)
└─ (8:17)

lvs returns

disk2 disk2 -wi----- 465,75g

lsblk (before decryption) returns

sdb                       8:16   0 465,8G  0 disk
└─sdb1                    8:17   0 465,8G  0 part

lsblk returns

sdb                       8:16   0 465,8G  0 disk
└─sdb1                    8:17   0 465,8G  0 part
  └─udisks-luks-uuid-.....-uid1000 (dm-7) 253:7    0 465,8G  0 crypt

mount /dev/mapper/disk2 /mnt returns

mount: unknown filesystem type 'LVM2_member'

fdisk -l /dev/sdb returns

WARNING: GPT (GUID Partition Table) detected on '/dev/sdb'! The util fdisk doesn't support GPT. Use GNU Parted.
Disk /dev/sdb: 500.1 GB, 500107862016 bytes 255 heads, 63 sectors/track, 60801 cylinders, total 976773168 sectors Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000
Device Boot      Start         End      Blocks   Id  System /dev/sdb1   1   976773167   488386583+  ee  GPT
Partition 1 does not start on physical sector boundary.