On the Campaign Main Page, Bob is shown to be a GM in the campaign by the blue "GM" shown on his icon

GM(Game Masters) are the ones who can control basically all aspect of a Campaign while in-game.

The Creator of a game is essentially a GM, but have exclusive access to a number of settings for the Campaign only they can access.

Players have much more limited options on what they can do, than the GM or Creator.

GM

A GM has access to the following:

Capabilities

Within the VTT

Full access to all Page b Layers
Create, Modify, and Delete Pages
Place images and tokens to the Tabletop from their personal P Art Library (not the Creator's library)
Assign and edit tokens
Create handouts & characters, assign their view and edit permissions
Move the Player Ribbon to different Pages
Add, Remove, and Play music from the u Jukebox
See GM Whispers and Rolls in q Text Chat
Create Macros and Token Actions
Modify r Dynamic Lighting on a Page if the game's Creator is a Plus/Pro-subscriber
Access to all in-game options on the Sidebar's y My Settings Tab in the VTT, except the Transmogrifier

Outside the VTT

Invite Players (anyone can share the unique join link to the game)
View and Edit API Scripts (Creator must be a
Pro
^{^info} Subscriber)
Manage the game's private forum, including sinking and deleting posts

Older Guide: Getting Started GMs

Promote player to GM

Promote someone to GM

The Creator or a GM can promote a Player to GM on the Game Settings-page. Hover your mouse over their icon, and select "Promote to GM". You should see that the player gets a blue "GM" on top of their icon.

A game can have any number of GMs, and multiple people can be logged in as a GM at the same time.

Rejoin as Player

The Creator or a GM can voluntarily disable their GM-status in a game, by in-game going to y My Settings-tab, and pressing the Rejoin as Player-button at the top.

By default, you always enter a game as a GM, if you are on it's main page, and either have to do the above each time, or you can right-click the Rejoin as Player-button and bookmark it.

Using the bookmark you can you can enter the game directly as a player, avoiding accidentally spoiling yourself something before you have gone to the button.

API

Some GM-only features can be indirectly granted to players through the use of different APIs:

ChatSetAttr -- can be configured to allow players to evaluate or edit stats on characters they don't control, useful for automatically reducing enemy HP, or granting healing/ temp HP to allies.
CharSheet -- A simple and easily customizable script allowing players to create their own character sheets, using the command !charsheet
Door Knocker -- Enables players to reveal/hide Dynamic Lighting sections such as doors. The GM can change doors from between locked/unlocked to adjust which doors the players can open. Speeds up DL management for GM as well.
- can be used for revealing hidden door, as well as making "windows"
emas -- Provides player !emas and !as commands. This is equivalent to the /emas, which is GM-only
MapChange -- has option to allow players to move between map pages, and GM can choose which pages are available/visible to them
Observer^(Forum) -- handy way to add/remove players as "Observer", who can see & control all player characters.
PublicSheet^(Forum) — create public, read-only versions of character sheets
Roll20 Audio Master (aka. Roll20AM) -- API commands to manage the u Jukebox, GM can grant selective access to individual players/tracks
Teleport -- provides a way for GMs or players to teleport tokens within,or between, map pages
TokenMod -- For managing token properties with chat commands. Usually players have limited control of their token settings, apart from editing the three bubbles.
- Includes setting that enables players to target tokens they don't control

There are also API that can be used to restrict players more that normal:

DryErase^(Forum) – An API that can stop players from drawing on the map, deleting any drawing instantly.
TokenLock – Allows GMs to lock/unlock the movement of all Player Tokens. - Useful for when you want to pause the situation and prevent players from accidentally moving forward.
MapLock - similar to TokenMod, but can be used for locking down any individual graphic/token on the map, so even the GM can't accidentally move things.
- Useful if you want players to have control over a token otherwise(see stats, edit/see name & notes), but stop it form being moved.