GMod

Achievement - Addict
You have wasted a year of your life playing GMod!

Achievement - Bad Friend
Kill 1000 friendly NPCs. Pretty much follow the same procedure as with War Zone and Innocent Bystander.
Create a nest of dynamite, and then spawn any NPC from the Humans+Resistance tab.

Achievement - Creator
Spawn 5000 props. This achievement goes hand-in-hand with "Destroyer".
So for instance, if you spawn 250 props, and then remove them, by the 5000th prop, you should have gotten both achievements.
Anyway, I wouldn't reccommend spawning more than 200-500 props in one map, depending on your computer. It's a bit of a grind, but doable.

Achievement - Destroyer
Remove 5000 things. Like I said in "Creator", just destroy all of the props you spawn.
To "destroy" something, you need to use the remover. The remover is found under Tools>Construction>Remover

Achievement - Dollhouse
Spawn 2000 ragdolls. Ragdolls are any characters in the Half-Life 2 (or any other game) spawnlist.
You can use this towards the Creator and also the Destroyer achievements.

Achievement - Friendly
Play with 10 friends. Obviously, you're going to need to have 10 friends on your friends list that own garry's mod.
Then, it's a matter of playing with them all. Not too much of a problem if you're a social person.

Achievement - Half Marathon/Marathon
These achievements are not as hard as they may sound.
Simply start up a game, making sure that you select the amount of players to be at least two. Then, just back off and come back 8 hours later!

Achievement - I'm the Real Garry!
This, quite possibly, is the hardest achievement in the game.
In order to get it, you have to be on a server with the real garry.
In order to do that, you will need to keep an eye on garry's official steam profile And join when he plays.
As he rarely plays nowadays, this could be nearly impossible.

(Link to his Profile is below)
http://steamcommunity.com/id/Garry/

Achievement - Innocent Bystander
Kill 1000 animals. Again, we're going to follow the same procedure that we did with War Zone and Bad Friend.
However, this time, spawn NPCs from the Animals tab.
I used Father Grigori, as he did not move, and of course functioned as a human, even though he's for some reason with the animals.

Achievement - One Day
24 hours of your life wasted

Achievement - One Week
One whole week of your life wasted

Achievement - One Month
One whole month of your life wasted

Achievement - 10 Thumbs
Your Workshop uploads got 10 thumbs

Achievement - 100 Thumbs
Your workshop uploads got 100 thumbs

Achievement - 1000 Thumbs
Your Workshop uploads got 1000 thumbs

Achievement - Mega Upload
These achievements will be quite difficult. I reccommend becoming experienced in gmod, learning how to add things. Then, after a few practice uploads, hopefully come up with a killer upload. After that, advertise it among your friends, and get them to rate it!

Achievement - Menu User
Open the spawn menu 100,000 times. This achievement will be quite the grind. This is where the achievement viewer will be very helpful.
As you should (hopefully) know, to open the spawn menu, press "Q".
I reccomend watching a movie or something while garry's mod is running, and repeatedly pressing "Q". To do this, startup your movie, press ALT+TAB to navigate back to garry's mod. You can put the window in the bottom right corner, so it shouldn't distract you at all. Then just keep opening the spawn menu. It might take a few movies, but you get the picture.
or If you want to make it easier, just go and download Menu User Tool on the workshop.
Here's the link to download Menu User Tool.

http://steamcommunity.com/sharedfiles/filedetails/?id=166775488
After downloading Menu User Tool, Go to Spawn Menu --> Weapons --> Others, You will find the Menu User Tool, Use it and let it go on.
You will get this Achievement done within 3-5 minutes.
After you get the Achievement, I would recommend you to Disable it and Uninstall it As Soon As Possible.

Achievement - Startup Millenium
Startup Garry's mod 1000 times. Not much I can say here, it's pretty self-explanatory.

Achievement - Time Waster
Accumulating 525,498 minutes on garry's mod is going to be a challenge, and will probably be the last achievement you'll get, if you do. The only thing I can suggest is to leave garry's mod running on an unused computer 24/7. Make sure that the game is not paused, or the Steam Community tab is not up.

Achievement - War Zone
Kill 1000 Baddies. An easy way to do this is to first place a large amount of dynamite in a small area, and then spawning, say,
200-500 zombie torsos, then blow them up. Rinse and repeat.