How use toolgun ?

Tools
Tools are used for construction. With it, you can weld (or stick) props together, rope one prop to another, give props another color or material and remove props. Tools can also be used for posing purposes.
For a list of tools, please scroll to the bottom of the page.
Using tools

1. Select your tool. Open your spawn menu by holding down q and click on the tool you want to use. Here, we'll be using the Winch tool.
   
2. Configure your tool. Most tools come with one or more settings; you can set these up however you like! It's worth noting that you can type a number into the slider boxes.
   
3. Aim at the first prop and lmb click!
   
4. Aim at the second prop and lmb click!
   
5. Sit back and admire your mastery of roping things together.
   

Source: Garry's Mod WIKI

Notes