Moved to the "Support" section.
Every game is different. Every game engine stores models and textures in a different file format. That's why for each game, the process is different as well. Usually in order to extract models and textures from games, you need to rely on other people to reverse engineer the game files and create programs for you to use. Depending on the game, someone may have made tools to extract those files. Sometimes no tools exist. If you want to extract files from a specific game, you're best off looking at the game's files and searching google for those filetypes. Note that some of the program's that you might find are not as easy to use as others and can require quite some technical knowledge. Once you've got the models and textures into a 3d modeling program like 3ds Max, Blender or Maya, you can export it to Garry's Mod or Source Filmmaker using this guide