These types of airsoft guns are obviously powered by gas. Normally a pistol and some types of GBB rifles have gas filled mags but that is more common on a GBB pistol. Normally people that use GBBR use a gas tank either in there back pack or on a belt around there waist with a cable connecting to the gun. There are two types of gas. One is green gas and the other is carbon dioxide.
Advantages of GBBR
These guns have an extreme fire rate compared to AEGs. The only other advantage I can think of would be that you can change your FPS of the gun.
Disadvantages off GBBR
First thing would be that you can change the FPS of the gun. I know i said it was an advantage but for that very reason alot of airsoft fields banned GBBR guns because they have a set FPS limit and you can change that on your gun whenever you feel like it so that puts other players at risk. The other disadvantage of GBBR is the fact that you are now dealing with a lot of seals and the maintenance for a GBBR is triple to what an AEG. Also a GBBR in the hands of a beginner is bad because they do not have any experience in gun maintenance and GBBR rifles are also extremely expensive compared to AEGs