Agree with BillT. We don't know what level of restoration you have in mind, so it's hard to advise. From the two pics, your tailgate is already in daily driver quality. If you want a higher level, you have several choices.
1. Buy a good used donor.
2. Fix the existing one.
3. A brand new tailgate with correct lettering is $350/$310.
Click Here....or Here. If you are wanting perfect, you can't get there cheaper than that, unless you do all the body work yourself.
A no bondo body man will charge more than $350.....and I can't even see the inside of your gate.
The "GMC" lettering was not modernized on the Fenderside, only on the Wideside. The Fenderside tailgates are the same from 55.2-62.