What is the value of a Gibraltar One Pound coin with a skull on the back?

The coin you describe is a general circulation Gibraltar One Pound coin commemorating the discovery of a Neanderthal skull in 1848.

These coins are still in circulation so, unless they are part of a Proof or Uncirculated mint set or are individual Proof or Uncirculated coins and in absolute mint condition, they are worth One Pound (GBP) in Gibraltar.