The G2 size is standardised and refers to the dimensions of the refill. G2 refills are available from many manufacturers and commonly contain oil based inks, but there are also many gel and rollerball refills in this size.
G2 refills have a diameter of 5.8 mm and a length of around 98.1 mm. The front section has a length of 23.2mm. Most pens accepting G2 refills will work with refills from different manufacturers, despite small differences in refill dimensions. Some retractable pens rely on the G2's sculpted plastic end for their retractor mechanism.
G2 refills commonly have a metal or plastic casing. Metal casings have thinner walls and allow a higher ink charge which results in a higher writing length. Oil based ink also allows a higher writing length than gel refills. Generally speaking gel refills have a writing length of several hundred metres. Oil based inks a writing length of several kilometres. 
Pens on stationery.wiki that accept the G2 refill
|Dimitri Pencil||1.15 mm lead|
|Mission To Mars||G2|
|Retro51 Classic Lacquer||G2|