Go to the developer console by hitting ~ (the key above the Tab key) and type the following codes for the desired effect.
bind x firstperson - Bind a key (x) to the command.
bind x thirdperson - Bind a key (x) to the command.
cl_drawhud 0 - Remove the information on screen.
firstperson - Go back to first person.
thirdperson - See the game from a third person perspective.
Complete the following tasks to unlock the corresponding achievement. To view your achievements and stats in Steam, select "Community", then "My profile", then "View all my games", then the game and view stats.
Addict: You have wasted a year of your life playing GMod!
Bad Coder: Experience 500 Lua programming errors.
Bad Friend: Kill 1000 friendly NPCs.
Ball Eater: Eat 200 balls.
Creator: Spawn 5000 prop.
Destroyer: Remove 5000 things.
Dollhouse: Spawn 2000 ragdolls.
Friendly: Play with 10 friends.
Half Marathon: Play on the same server & map for 4 hours.
Innocent Bystander: Kill 1000 innocent animals.
Map Loader: Play 20 different maps.
Marathon: Play on the same server & map for 8 hours.
Menu User: Open the spawnmenu 100,000 times.
One Day: 24 hours of your life wasted.
One Month: One whole month of your life wasted.
One Week: One whole week of your life wasted.
Play Around: Play a gamemode that isn't sandbox.
Play Multiplayer: Play multiplayer at least once.
Play Singleplayer: Play singleplayer at least once.
Popper: Burst 1000 balloons.
Procreator: Spawn 1000 NPCs.
Secret Phrase: Say the secret phrase.
Startup Millenium: Start Garry's Mod 1000 times.
War Zone: Killed 1000 Baddies.
Yes, I am the real Garry!: Play on the same server as Garry.
Easy "Secret Phrase" achievement
Press the Y to open the chat box during game play, then type rusty bullet hole and press [Enter]