GatherSage2 shows what level is required to mine, pick, smelt, prospect, mill, or skin a resource (vein, deposit, ore, animal, plant, etc). The tooltip shows all skills for which the resource can be gathered or used up (i.e. an herb that can be both picked and milled).
It shows your current skill, how many skill ups you've gained since you logged in, and the difference between the skill you need to work with the resource and your skill level.
It displays this information in the tooltip when you mouse over a resource in the world, minimap, inventory, auction house, mailbox, guild bank, and pretty much anywhere the item can be displayed. It also shows if the resource has a chance of yielding special items, such as stone, gems, or other herbs.
1. Mousing over a world item such as a mob, herb, or mining node sometimes resets the tooltip to the default game tooltip with no GatherSage2 information. There is little I can do to control this. A workaround is to mouse over the minimap to quickly hide the tooltip and try again.