Refers to Glass-On-Glass, or GonG. Standard water pipes (bongs) use a glass slide and a rubber stopper to seal off the chamber, but GonG is a more advanced system that involves precision laser glass cutting so the GonG pieces fit perfectly together, creating a more efficient means of smoking. GonG is also a way to "add on" to your bong, including ashcatchers, filters, diffusers, pretty much anything that is compatible. The most common sizes for GonG accessories are 14.4mm and 18.8mm(referring to the width of the glass opening). Glass-on-Glass bongs hit harder and smoother than conventional bongs.
Yesterday I bought a new GonG style bong from a local headshop, along with an ashcatcher and upgraded diffuser downstem.