GPT disk unaccessible lost HFS+ partitions

MBP early 2011 running OSX 10.11.6 3TB Seagate external HDD

My Seagate had all (not sure how many) HFS+ partitions working ok until one day it stopped mounting.

currently listed as /dev/disk3 and gdisk finds no partitions:

Type device filename, or press <Enter> to exit: /dev/disk3
Warning! Read error 5; strange behavior now likely!
Warning! Read error 5; strange behavior now likely!
Partition table scan:
  MBR: not present
  BSD: not present
  APM: not present
  GPT: not present

Creating new GPT entries.

ommand (? for help): p
Disk /dev/disk3: 8089950 sectors, 3.9 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 9FBCC48E-F444-4824-8A65-37982CD1297B
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 8089916
Partitions will be aligned on 2048-sector boundaries
Total free space is 8089883 sectors (3.9 GiB)

Disk size is 8089950 sectors (3.9 GiB)
MBR disk identifier: 0x00000000
MBR partitions:

Number  Boot  Start Sector   End Sector   Status      Code
   1                     1      8089949   primary     0xEE

Verifying only scans what it can find:

No problems found. 8089883 free sectors (3.9 GiB) available in 1
segments, the largest of which is 8089883 (3.9 GiB) in size.

I tried a hexdump:

MBP3OS:~ tivadark$ sudo dd if=/dev/disk3 bs=512 count=1 | hexdump -C
Password:
dd: /dev/disk3: Input/output error
0+0 records in
0+0 records out
0 bytes transferred in 0.151342 secs (0 bytes/sec)

I am afraid to use the gdisk r options. Can someone advise how I can go about restoring my partitions?

Simply put, the drive sounds like it's died. An input/output error means that the data from the drive isn't making sense, or there simply isn't any data coming back.

You can try the various 'fixes' listed here however these are for all recovering data, not 'fixing' the drive - I very much doubt you'll be able to fix the drive.