The default playermodel of Garry's Mod is this Half Life 2 character.
Explanation
The answer is incorrect. The default player model in Garry's Mod is not Isaac Kleiner, but rather the character "Gordon Freeman" from the game Half-Life 2. This is reflected in the game's lore, as players take control of Gordon Freeman in the Half-Life series, and his iconic appearance is used as the default in Garry's Mod.