The Gecko settlement is the first part of Gecko encountered by the Chosen One. It is the "downtown" of Gecko, with the living areas, bar, and administration buildings.
It has a large open square with homes and the two largest buildings, the Administration building and the Harp bar, ringing it. The town well is located nearby, close the the entrance to the reactor. It has access to the junkyard near the brahmin pens, and the main Reactor doors.
The Harp is a bar run by Wooz situated along the side of the reactor. It sells irradiated liquor such as Gamma Gulp Beer. Wooz is also Gecko's resident Tragic addict, and will play with the Chosen One if asked.
The Manager's office is where Harold and his assistant Lenny reside, and run the small settlement. The have problems with Vault City, but cannot get near enough to bargain for the needed part, as they shoot ghouls on sight. Harold is a very trusting individual, considering his life experiences, and Lenny can be recruited as a companion.
Here one should talk to Harold before entering the powerplant, as there must be some parts that need fixing... and Harold can explain what exactly needs.
Also, a requsition form can be found here for taking (legally) stuff from the supply room in power plant.
Gecko settlement appears only in Fallout 2.