You will want to aim for the creature's head. The rifle-wielding sharpshooter must follow a different technique, because any bullet that hits a gnome will take more gnome than not. No, the sharpshooter's technique is subtle, efficient, and more useful than any well-placed hit. A concussion shot may be taken at a treed gnome. Although treeing a gnome may seem like a lot of work, the trouble is definitely rewarding, as a gnome bagged live via concussion is certainly more valuable than a dead one.
Gnomes are excellent climbers.
Forced to choose between the hound's jaw and the oak's branches, every gnome will pick the latter. Because gnomish bellies are fat and gnomish arms short, a treed gnome will lie flat and still against the trunk, gripping the bark tightly with its fat little fingers. Take your opportunity here. Fire just to one side of the gnome's head, hitting the tree. Even a strong gnome will fall immediately and can be collected and bagged after it hits the ground.