How to Speak in Garry's Mod:

Open Chat: The most common way to communicate with other players in GMod is to use chat. To open the chat, press the "Y" key by default.

Enter Your Message: After opening the chat, you can enter your message. Just press the keyboard keys to write your message.

Send Message: Once you have written your message, press "Enter" to send it. Your message will then appear in the chat for other players to see.

Using Chat Commands: In addition to standard chat messages, you can use special commands by starting your message with a "/" followed by the command. For example, "/me dances" will display "YourName dances" in color and italics to show an action.

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Speak in voice chat:

You need to press your "X" key which is normally the key first put as the basic command.
You can also press your mouse button "Mouse 3". You can always change the key in: Settings > Keyboard > Enable voice communications.

How to Walk in Garry's Mod:

Basic Controls: GMod generally uses the same basic controls as other FPS games. To move, use the following keys:

W: Move forward
A: Go left
S: Back
D: Go right
Space: Jump
Crouch: To crouch, use the "Ctrl" key by default. This can be useful for hiding or sneaking into tight spaces.

Sprint: To run faster, hold the default "Shift" key while moving. This allows you to move faster, but also uses energy.

Swim: If you're in water, use the standard movement keys to swim, while holding the "Shift" key to swim faster.

Keep in mind that default controls may vary depending on game settings, so you may want to check your control options if you've made any changes. Additionally, Garry's Mod is a very modular game, and controls and features may vary depending on addons and game modes used. So be sure to read the specific instructions for the server or mode you are using to maximize your gaming experience.

So I hope this was useful for you.

Sincerely,
Normal guy.