No free trial of Garry's Mod is offered on the official Steam website as of 2015. However, several unreliable websites offer versions of a free Garry's Mod trial that are known to almost never work correctly. It is suggested to buy Garry's Mod on the official Steam website for $9.99.
Sometimes, the Steam website puts Garry's Mod 10 on sale for $6.99 alone and other times makes it $4.99 if purchased with other highly demanded games such as Team Fortress 2. Garry's Mod 9 is available for a free download, but it is not supported by Garry anymore and has no more servers as all players moved on to buy Garry's Mod 10.