It is a relatively new skill (a year old or so). Although it is listed among the skills, it is really more like a minigame or activity, in which you have to find the exit of an underground dungeon to get points.
from the dunginering tutor
Yes.
drink the antidote
The deepest level floor available to non-member RuneScape players is floor 35. Floor 35 can be accessed by players after reaching level 69 Dungeoneering (about 650,000 experience in Dungeoneering).
you get it from dungeoneering for 34k or 44k tokens
Red and Gold band.
You can only get dungeoneering tokens by getting experience in dungeoneering - that is, by completing dungeons, alone, or in teams of up to 5 people.
In the Dungeoneering minigame. Where they spawn is random.
The highest level for all skills except Dungeoneering is 99. Dungeoneering can be trained up to level 120.
If by Gravity Shortbow you mean the Gravite Shortbow, it's a reward bought from the Reward trader for 40,000 Dungeoneering tokens outside of Dungeoneering. It can't be brought into the dungeons and isn't an item in the dungeons.
Rewards, at the Reward trader south of Daemoneim.
A new skill has just been released: Dungeoneering.