Gathering

Gathering is divided into Hunting, Mining, Woodcutting and Quarrying.
To level up any of these, all you have to do is gather resources. Every attempt will give experience which contributes to your gathering level. The higher your level, the more resources you'll receive per attempt.
You can also upgrade the tools you use. Doing so will give you 10% more resources per level of rarity.
Upgrade cost is the following:

Bonus:	Gold cost:
+10%	25,000
+20%	250,000
+30%	2,500,000
+40%	25,000,000
+50%	100,000,000
+60%	250,000,000
+70%	500,000,000
+80%	750,000,000
+90%	1,000,000,000
+100%

A calculator for resource gains:
Heroes RPG Tradeskill Calculator

There are 2 hunting and 2 mining options.
For hunting you can choose between "Hunt For Food" and "Hunt For Leather". The leather is used for the Leatherworking skill.
For mining you can choose between "Mine Essence" and "Mine Metal". The metal is used for the Forging skill.
For woodcutting you can only choose "Chop Wood".
For quarrying you can only choose "Cut Stone".

These resources are all used for upgrading clan buildings.

Experience is directly based upon resources gained (10 x resources gained = experience gained).