boulevard, street, town
街 centers on a main road in a populated area: a boulevard, a street, or by extension the town itself that such streets define.
街 combines 行 (to go, movement) with 圭, which likely contributes the sound. The compound originally referred to a crossroads or a main thoroughfare in a settlement.
The outer 行 suggests movement, and the inner 圭 looks like stacked earth or blocks. Picture a paved road lined with blocks of buildings: a street or boulevard in a town.
For ガイ, imagine a guy walking down a busy boulevard: guy -> ガイ, and he strolls along the street.
street; quarter; district; area
highway (esp. one existing from the Edo period); main road
town area; urban area; metropolitan area; built-up area
(on the) street
urban areas; the streets; town; city
street corner
carrying out (political) propaganda activity on the streets
road; street; avenue
street light; streetlamp
underground shopping center; underground shopping centre
Wall Street
entertainment district; amusement area; pleasure quarter; red-light district
red-light district; prostitution quarter; geisha quarter
area zoned for urbanization (urbanisation)
food section in a department store with well-known tenants
Appian Way
shopping arcade
Occupy Wall Street (protest movement, 2011-)
business district; office complex
street where many shops and businesses are closed (e.g. due to an economic slump)