There are many accesories for gas systems, and sometimes many useless threads on them. so i will make a post thats covers them all.
Gas Thru Stock
They are mounted on your bottom-line screws and with a remote attached to your air system. Useful for keeping your gun steady, but are you willing to pay? Not that common anymore.
Shown at the top of the picture, with a P stock in the middle (with a bottomline attached to a built in drop forward) and a plain T stock at the bottom.
A selection of remote lines can be seen here.
Remotes screw into you ASA or drop and put the tank in a "remote" location. They come in stainless steel hose and coiled. The coiled is better, because it is more flexible. The remote make for faster snap shooting and faster breaks. They also act as an Expansion Chamber. All or most are tested to 3000psi.
Turns your pin valve co2 or preset nitro tank into an on/off. Adds about 1.5in to your tank though.
This one is made by Smart Parts and is widely available.
A selection of Regulators can be found here.
They regulate the amount of pressure that gets into the marker. When used with CO2, they will work best with an anti-syphon tube installed in the bottle.
This is the Shocktech version of the Custom Products regulator - a very good choice for it's relatively low price.