As of 2015, there is no free trial version of Garry's Mod. However, Garry's Mod Version Nine is still available for free download at moddb.com. Moddb.com is a trustworthy website that hosts various modifications of different games, although care must be taken when downloading files from the Internet.
Version Nine is single-player game only and is an older, unsupported model of the game. It lacks access to the variety of modifications created by the Garry's Mod community. It is still available to raise interest in the official, updated version. As of 2015, Garry's Mod Version 15 is available for purchase on Steam.