Gable end:

The triangular shaped section of a wall built up to the peak of a roof.

Gable wall:

An end wall which extends up to the apex of the roof, with a triangular shaped section at the top.

Gully (or gulley):

The collection chamber at the base of a downpipe or set within an area of paving, covered with a metal or plastic grating, conveying rain or waste water into the underground drain,
or
a drainage channel cut into the ground, often with a protective cover or grating.

Gusset plate:

A timber or metal plate fixed across the joint of a roof truss or other framed structure to provide additional strength.

Gutter:

A horizontal drainage channel located at the base or edge of a roof.