In the gamemember

Settings

Where you switch each part of the addon on and off and adjust how it behaves.

In your game: Settings

What it does: Five categories. General has My Profile (the roles you are willing to play), window scale, the Modules that switch each tab on and off, quick access, general options, chat and mentions. Loot has the loot system and the master looter's rules. Raid has the attendance penalties. Officer has the officer ranks and maintenance. About has diagnostics, backup and the links.
Who sees it: Everyone, and it cannot be turned off. Loot and Officer are officer-only; in Raid, a member sees the penalties but cannot change them.
Where to find it: In the window (J or /gos), the last tab. It is also in the minimap button menu (right-click).

The categories are General, Loot, Raid, Officer and About. Loot and Officer only show for officers.
In Modules, everything comes on. Now, Roster and Settings have no checkbox: they cannot be turned off.
Unticking a tab removes it from the window right away. Unticking a background piece, such as the Raid Tracker, only takes effect after reloading the interface, and the addon warns you.
UI Scale goes from 80% to 120%, in steps of 5.
Guild info in item tooltips has three modes: always (the default), only when holding Shift, or off.
Guild button opens Guild OS comes on: J and the guild button open Guild OS when you are in a guild. Turned off, they open Blizzard's window, and Guild OS stays on the minimap button.
In Officer › Officer Ranks, ticking a rank ticks all above it, and the Guild Master is always an officer. Only an officer changes it, and the change applies to the whole guild.

The loot system chosen in Settings › Loot applies only to your game: each officer picks their own, and a member stays on /roll.
Turning off a tab does not turn off what it does behind the scenes: without the Raids tab, raids keep being logged.
Changed the UI Style and nothing changed? It only applies after reloading the interface.
Guild OS's own shortcut in Key Bindings comes with no key: what opens the window by default is J, through the guild button.