69

Note the number 69 on the runway

The 69 sex position is an innuendo recurring throughout the GTA series. This page lists some of its appearances:

The game GTA London 1969
The play 69th Street advertised in Grand Theft Auto III
Area 69 military base in San Andreas
Runways at all airports in the GTA III Era being numbered 69
Shipping containers bearing the number 69
69 Cents used as prices and the name of a chain of discount stores
The San Fierro 69ers American Football team, a reference to the San Francisco 49ers
On the side of the aircraft carrier in Easter Basin Naval Station in San Fierro
The 69th Street Diner in Grand Theft Auto IV
On the side of buses in Liberty City