Here are some games on the Source engine that you can use to add content to Garry's Mod:
Team Fortress 2: This popular multiplayer first-person shooter offers a wide range of character models, weapons, and props that can be imported into Garry's Mod. You can create your own unique scenarios and gameplay using the assets from Team Fortress 2.
Left 4 Dead 2: If you're interested in creating zombie-themed maps or scenarios, Left 4 Dead 2 is a great source of content. You can import character models, infected zombies, weapons, and various props from the game to enhance your Garry's Mod experience.
Counter-Strike: Source: This classic first-person shooter provides a plethora of weapon models, player models, and props that can be used in Garry's Mod. Whether you want to create a realistic military-themed map or simply add more weapons to your arsenal, Counter-Strike: Source has you covered.
Day of Defeat: Source: If you're a fan of World War II-themed content, Day of Defeat: Source is a fantastic resource. You can import character models, weapons, and props from this game to create immersive war scenarios in Garry's Mod.
Half-Life 2: While you mentioned that you don't want content from Half-Life, it's worth noting that Half-Life 2 and its episodes offer a vast array of assets that can greatly enhance your Garry's Mod experience. From iconic characters like Alyx Vance to unique weapons and vehicles, Half-Life 2 provides a wealth of content to explore.
Remember to check the Steam Workshop for Garry's Mod, as there are countless user-created addons and mods that can further expand your options for additional Source games content.