I am making a gnome ranger for my Saturday game and I've decided, after 10 years, to finally take a shot at a ranger. He isn't going to be a very physical ranger and will depend more on his gnomish equipment, traps, and animal companion to help him out.
Strength 10
Dexterity 13
Constitution 16
Intelligence 12
Wisdom 18
Charisma 15
As far as a direction I was thinking ranged since he will utilize crossbows and other ranged weapons. Character wise he's much more of a vocal protector of the woods and often will try to talk down a conflict over directly fighting. When it comes down to it it is more his alliance with nature and spurring them to greater deeds that is his method of combat.
If your stats aren't locked in, I'd put that 18 into Dex instead of Wisdom, even if you don't plan to fight much. As a ranged character, you'll want at least a few ranged feats (point blank shot, precise shot, etc) for those times when words don't hurt enough. Rangers don't have many social skills, so you'll be leaning heavily on Intimidate (unless you spend a trait on Diplomacy or Sense Motive). Max that out, along with Stealth, Perception, Ride, Survival, and Handle Animal (why limit yourself to only one animal ally?). Make sure you know the languages of the creatures you're likely to encounter. Heck, add those creatures as favored enemies to get miscellaneous bonuses when talking to them.
I agree with Marius, Rangers are weak casters and full BAB fighters. Dex will serve you much better than Wis. Small rangers are exceptional at using there animals as mounts and through feats getting full attacks. Honestly, wisdom at 10 is fine, you'll want to get it to 14-16 with magic items, but it doesn't have to be immediately.
I do in general agree, you could use more DEX (14 -16 at least, I'd try to get 16) to make full use of your bows, and I recommend Longbows over Crossbows, in general. That way you always have a way to contribute to combat if the GM throws something particularly nasty. At the very least I recommend trying to get Point Blank Shot, Precise Shot, Deadly Aim, and Rapid Shot within the first 6 levels (using bonus feats to help).
That said, to make full use of your concept- take Boon Companion at level 5 to make your animal companion as strong as possible. I recommend Badger, but only because the idea of a Gnome Ranger riding in on his Raging Badger mount is amazing. If you aren't too attached to spells (and don't get me wrong, spells are marvelous), try the Trapper archetype- I'm not sure how well it'll work, but it's better than trying to make traps using the normal rules. The way the character will turn out is a character who is good with a bow, has a decent-not-spectacular animal companion, and can lay quite a few traps per day. The build won't fully come into it's own until level 5, when you get Boon Companion and Traps, but Ranger is something of a late bloomer in general.
As for being a diplomatic character, I recommend taking a trait to get Diplomacy as a class skill, otherwise Ranger's 6+INT skill points per level should allow you to play your character just fine without feat support.
This character won't be quite as good as a normal Ranger, and I honestly have no idea how the traps will go, but it should be mostly functional. The key thing is just to make sure you have some baseline skill with that bow, and that's going to require most of your feats at first.
I can understand wanting to play a gnome who isn't very physical and is more of a vocal protector and will try to talk down a conflict. But why a ranger? That does not fit the characterisitcs of the ranger class. Rangers are a full BAB class so they are very physical. Rangers typically are not vocal. Ranger's only advantage in talking down conflicts is Wild Empathy.
Playing a character who works against his class abilities is not fun. You said that you wanted to finally try out ranger, but this idea would not be a fair trial of the ranger class.
I recommend choosing another class. Hunter and Druid have a good animal companion, with Druid benefiting more from Wisdom if you want a wise gnome. Talking down an opponent relies on charisma, and charisma favors a spontaneous caster class, such as Bard, Oracle, or Sorcerer. If you don't want high Charisma, a few classes make Charisma-like uses of Wisdom, such as Sensei Monk.
With Diplomacy as a class skill (through a trait) and full development of Diplomacy, one can become pretty decent at talking things out. No better than any other character, but much better than a character that ignored diplomacy. If your goal is to end every fight before it starts, that won't be enough. But if your goal is to occasionally talk people out of doing things that are stupid, you should generally have enough to make it work every so often. Especially if TC doesn't fix his stats and has 15 CHA, but even with 10 CHA it's a +4 at level 1, which is not great but will eventually get better and better, and Diplomacy DCs don't scale well. It's also enough to succeed easily on Aiding other Diplomacy attempts. In other words, don't be the Face, support the Face.
As far as whether Ranger is the best class for the job, really it depends how much TC wants his traps. If it's truly important, then Ranger is the best because of the Trapper archetype. If not, by all means, play a Hunter instead for the better animal companion. And because of the emphasis on an animal companion, you do have a lot more leeway to play with a weaker build.
There's an archetype for sorcerer that is nature themed and gives you an animal companion. That would be a good match for a gnome. You can use your illusions to help ward off enemies instead of fighting.