This was the most challenging achievement i have had to do, but also the most fun. I extremely suggest anyone with this game to try.
The beginning is pretty easy, just bring him with you, and make sure he's not forgotten. When you get to the vort chamber, just place him in a corner (not right up against the elevator, he can fall in and you'll never get him back) and when you get back, same thing.
Before big battles, find a safe place where he won't be touched and pick him up afterward. In the area where you have to get to the car, before you go in the first room, shoot the gnome over the wall you're trying to get around. Don't worry, he'll still be there.
When you're driving, put him at your or Alyx's feet, that way you can see him if he falls out and quickly get him back and continue. Just make sure not to drive fast. When you go into the house with the big floating green worm-thing (i forgot what it was called), make sure you don't place gnome in the car, cause the car gets teleported and you'll be wasting time searching for him on the ground.
When you get to the part with the hunter-chopper following you, place the gnome at the beginning of the train yard. Continue until you get the first load, then go back and you can safely bring the gnome forward with no chopper. Once you get back past the load, I kept shooting him forward, going to where he went, picking him up, and repeating. Once you kill the chopper, it's pretty easy, just keep him with you.
Just have fun and name the gnome if you want.