GMod has a free recorder built into the console. To record a movie, open the console with the "~" key. Type "Record [Movie]" and press the Enter key. The video will begin recording. When you want to stop the recording, open the console again. Type "Stop" and hit Enter.
The video will be saved as "Movie". To convert the recording from Gmod's native format to AVI, open the console again, type "Startmovie [Movie] " and hit Enter. When prompted, choose "Uncompressed" and then type "Playdemo [Movie]". The movie will now convert to an AVI video. The video will be saved in the GMod directory. For example, "C:Program FilesSteamSteamAppsusernameGmod ". If for some reason Gmod's native recorder does not work, the next best option is to use a dedicated game recorder. A great free option is Open Broadcasting Software. Open Broadcasting Software enables you to record everything that can be seen on screen. Fraps is one of the most popular game recorders, but purchase is required if you want to record more than 30 seconds of video at a time. A quick Google search should lead you to the respective download sites for these programs. Before recording Gmod, it's important to ensure that you have plenty of free RAM and processing power. Game recorders can slow the computer to a crawl due to the intense rendering that is required.