Use "sv_autojump 1" to enable autojump for server first.

What does it do?

Allow players auto jumpping by holding "SPACE". You can auto jump if the server installed this addon.
This is not a client-side script and should work on any gamemode.

What is Bhop?

BunnyHop Wiki

Server Settings

Change "sv_airaccelerate" of your server to 100 or higher.
If you can't autobhop on the slope, make sure the "sv_sticktoground" set to 0.

How to disable jump speed limit in Sandbox

We all know Sandbox has a jump speed limit and cannot bhop when crouching, it will slow you down when Bhopping too fast. It is used to prevent players Bhopping at a ludicrous speed.

To disable the speed limit in Sandbox please follow these instructions:

Don't do this if you have no experience.

1.Go to the Garry's Mod folder "\Steam\steamapps\common\GarrysMod\garrysmod\gamemodes\sandbox\gamemode\player_class"

2.Use Notepad++ to open "player_sandbox.lua" (Please make a backup just in case.)

3.Find line 183, change "0.5" to "0.25". (This will reduce the speed for each jump, because it gets boosting too much in sandbox.) Skip this one if you want more speed.

4.Find line 189~192, delete it.

5.Save.

6.Now you can go tell your friends you are a pro gamer.