The best way to see the Rock is to take the Taxi tour, it takes about 1 1/2 hours and the price includes entrance into the upper rock ( it's a nature reserve ) and the cave which has a theatre inside its well worth a visit, also the siege tunnels, you will see plenty of monkeys, and great views of Africa on a clear day you can see right to the Rif mountains in Morocco. The price is 25 Euros per person and it's well worth it. If you make your own way up its about 12 Euros just to get in the nature reserve, and then pay extra to get in the cave and pay again to get into the tunnels. You can catch the taxis from Casemates Square. What ever you choose enjoy Gib.
Ian.