I finally have it, it is really a pain in the ass to get. Especially when the Gnome is taking a ride along with the Helicopter behind my tail.

Screenshots
Achievement Stats

I can't get my lazy arse up to complete it.

I tried, when I came back after grabbing the vortal essence he disappeared.

I'm currently trying to do it. Managed to get past the helicopter chase, but I lost it (damned grenade!) when I was about to take down the turret thingy afterwards, and off course I had forgotten to save for a little while, so meh. Going to try get the little fellow further when I get a new graphic card (fan stopped on my current ).

On the Helicopter part, driving slow will only have yourself killed faster. The way I did it, I use the gravity gun to throw my gnome ahead, then drive there and pick up him and repeat. Not a good idea despite what the Helicopter throws at you along with Zombines ahead.

However, if you reach the end of the Helicopter chase, without destroying the locked gate (that would suddenly destroy your car), the Helicopter will not bother you anymore. And you are free to catch your gnome via car and come back, then by having your car demolished, you can now deal with the Helicopter again.

If you are referring to the Vortigaunt picking up a larva, the gnome should remain with Alyx as long if he isn't sitting by the bottomless pit of where you use the lift.

FUN FACT:
Not closing the little hatch door on the rocket which you put your gnome in will make you not get the achievement when you launch the rocket because the gnome will fall out of it.

My friend told me that because it happened to him.
He ended up redoing the whole game. I asked "why didn't you just reload a save?" I still can't get a straight answer out of him.
I believe that he thought that he would get the achievement after the credits. The fool.

Another good reason to know if the achievement wasn't unlocked is because Magnusson probably cleared the rocket out, unless your friend saw the Gnome fell out during the launch sequence.