gl is short for "geolocation" and sets the country a Google search is treated as coming from. gl=ca is Canada, gl=us the United States, gl=gb the United Kingdom.
It is a coarse signal. A country is not a city, so gl alone tells Google nothing about which street or metro you are in — it leans results toward a country's version of the index and its local businesses, and that is all. For city-level precision you need uule on top of it.
gl is also independent of language. Setting the country does not set the interface language; that is hl's job.
See how gl changes real results: compare Google from any two locations.
Deeper read: Why your keyword has no single Google ranking