Here you can find a description of the basic use, applications, and nuances of every tool in Garry's Mod used by the Tool Gun. There are 4 types of tools: Constraints, Construction, Poser and Render.
Constraints props and ragdolls with a hydraulic that will expand and contract repetitively whenever you press the bound key.
Nail
A simpler form of a weld. You click on a thin prop that has something behind it and a nail is shot through the prop into the object or brush behind it, holding the prop in place.
Pulley
Constrain two objects with a rope passing through two pulleys.
Joins two props together no matter where they are. They cannot be moved individually afterwards.
Easy Weld
The same as Weld, but first moves the objects so that they are touching.
Winch
A rope, the length of which can be changed using the numpad.
Construction tools[]
Balloons
Attaches a balloon prop that either pulls upward like a balloon or downward like a weight
Buttons
Simulates pressing a key on your numpad. Useful when you want other people on a server to be able to use your contraptions.
Duplicator
Copies an entity or group of entities if they are fixed together.
Dynamite
Timed explosives, on the press of a button you can detonate it.
Hover-Ball
Simply add a hover-ball to anything you want to float. By using bound keys on your numpad you will be able to control the height of your hover-balls.
Ignite
This tool can set any prop and certain NPC's on fire. Right clicking extinguishes the fire. If the prop is breakable, this will eventually break it. This is useful for making scenes and comics.
Lamps
Spawns a dynamic light that shines in one direction.
Light
Spawns a dynamic lightbulb that shines in all directions.
Emitter
Emits effects such as shell casings or sparks.
Magnetize
Turns a prop into a magnet, clinging to all other props. Has the side effect of making props unbreakable.
No Collide
Stops two objects from colliding. Useful for preventing spazzing (aka the black hole effect).
Physical Properties
By adjusting the physical properties of an object you can easily change how an object interacts with other objects, as well the world. For example you can change the friction or buoyancy.
Firing this tool at anything that you spawn from the menu (props, ragdolls, npc's, etc.) will instantly delete it. Great for getting rid of dangerous npc's or spazzing constructions.
Thruster
Thrusters provide force backwards and forwards from the direction of the thruster. Thruster power ranges from 0 to 10000, but highly depends on the mass of the model you choose.
Wheel
Attaches wheels that can be controlled with the numpad. Similar to the Motor constraint.
Physics Gun
Allows you to manipulate any spawned prop.
Posing tools[]
Eye Poser
Changes the direction a ragdoll's eyes are looking.
Face Poser
Modifies the expression of an NPC or ragdoll's face.
Finger Poser
Allows you to move a ragdoll's fingers.
Statue
Freezes a ragdoll in their current pose. Useful if you want to move a ragdoll somewhere without messing up its pose.
Inflator
Inflate or deflate portions of a ragdoll.
Visual tools (Render)[]
Colour
Colorizes entities.
Material
Changes the material of an entity.
Cameras
Places viewable cameras.
Paint
Sprays sprites across a surface.
RT Camera
Places the render target camera. This is a server-wide camera everyone can see.