Half-Life 2: Campaign & Team Fortress 2: Invasion are two of the most popular games you can play on GMod. Here is a brief overview of both:
Half-Life 2: Campaign
Half-Life 2: Campaign is a Garry’s Mod game mode that lets you play without getting the sandbox and weaponry. You can also play it with your friends. Half-Life 2: Campaign does not offer sandbox features neither does it include maps from Half-Life 2.
Team Fortress 2: Invasion
Team Fortress 2: Invasion is a cancelled TF2 version which was in development between 2001 and 2002. It has undergone various design modes from Humans vs Aliens to the War of the Worlds. However, this was not publically revealed. Its gameplay is different from any TF2 game. The objective is to get more resources before a round ends. Resources are used to build objects for your team. Spawn vehicles such as Motorcycles, Wagons, Battering ram, Siege tower, alien-exclusive tripods referred as striders or walkers at the vehicle bay!
The buildable objects include Sentry guns, Powerpack, Buff Station, Healing Station, Spawn Station, and the Resource Pump. The Powerpack was used as a power source for objects and could power up to four objects at once. You can also build objects such as sentry guns, buff station and resource pumps. The Powerpack is used to power four objects at once. You can also seize control zones within a map which would become your territory to use and build in. The game also features environmental hazards such as Meteors striking a map, lasers and rockfalls. Every round is separated in 3 acts based on the time set according to the game’s map.
Check out Garry’s Mod Game Modes Overview