Oh... well... Looks like they know about this... I think... And they care about this exploit ._.
After December 2015 Update, this exploit has been fixed, this is not working anymore :(
You may want to read some updates here : http://www.garrysmod.com/updates/
(read the most bottom of [FIX] tag on December 2015 update, it is written on it)

• No VAC ban (VAC ban is only when you modified the EXEs and DLLs, or using softwares like Cheat Engine. For more information, search on google :D )
  
• I don't responsible if you get banned on achievements sites like AStats, etc.
  
• No Achievement manager or similiar software
  
• This... will make you as a cheater (how the hell you can get the "Addict" achievement without a year??)
  
• This guide is serious, I'm not trolling you :v
  
• Use this guide AT YOUR OWN RISK

The first step is simple, start up the game...
And :

1. Make sure you are connected to the internet and your steam profile status is online
   
2. Don't just launch the game, you must loading the map (any map, just don't let the game idle on start screen)
   
3. Err... Yeah make sure you are not single player, just create a private server (Start New Game -> from the "Single Player" dropbox, select "2 Players", and then check the "Local Server". This will block random people to mess up your game.

After you finished loading the map, open the console

You don't know how to launch the console? Win + R and cmd.exe

1. Pause the game
   
2. Open Options, then when you in "keyboard" tab, scroll to the bottom and you will see "toggle console" like the picture below :
   
   
3. Click that "toggle console" and then click "edit key"
   
4. Choose what button you want to toggle the console (I choose F10)

Note that "sv_cheats 1" in GMOD will not disable any achievements!
This code allows you to see how many minutes remaining before you will get the "Addict" achievement
You can see that every minute this "count increased" popped out, and maybe it is not same as your record on your Steam profile.
There is the reasons why that count isn't same :

• You pause the game while singleplayer
  
• Too much time wasted when the game try to mount all of your addons
  
• You may let the game idle without loading any map
  
• Your steam is offline when playing

After you understand how this achievement works, time to speed it up :D
Type this :
You can replace the 10 with how many times faster
This code will make your game 10 times faster

Before you idling your game, try to count them using your stopwatch, when it set to 10, it should pop out 10 times within a minute. If it not, try lowering the value
If you set on 9, you should see it pops out 9 times
If you set on 8, you should see it pops out 8 times
and so on...

You want get this faster? You can increase the value! Please note when I tried at 15, it just pop out about 12 times :(

When you are idling the game for this achievement, I suggest you to make the game use lower system resources (RAM, CPU, GPU) as low as possible

• Try lowering the resolution of the game
  
• "mat_fullbright 1" and "r_skybox 0" will boost the FPS (not really, or maybe little)
  
• Use power saving profile on your Windows (don't use if the achievement pop out slows down)
  
• Load the map that use lowest system resources

HAVE FUN!
:D

If you see bad grammar or something like bad english, please tell me and I will fix it :p *bad english*
Also, if you have more suggestions, tell me :p

If this works, don't forget to thumbs up this guide :p
If this not working... uh... yeah you can thumbs down :(