hard drive - 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  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?