The things to watch out for

This method just as any other is not exactly perfect. The most important issue is that there is quite a big chance that the gnome will fly out if you hit anything. I advise that you look at the gnome often and check if it wants to leave your car. Now this is not that big of an issue, what is a big issue is the loading zones. As you probably know many loading zones are placed at the end of the tunnels, the problem is that when you load in again the car jerks a bit, in my experience this has almost without a fail caused the gnome to fly out right into the tunnel. This might not seem like that big of a deal but it's quite hard to find the gnome when this happens, also i'm fairly certain that you can straight up lose the gnome by launching it into the loading zone. The way to combat this is driving into the tunnels in reverse.