Gauge is the thickness of a stainless steel sinks. The LOWER the gauge, the thicker the stainless steel.
20 gauge is considered low-end and pretty cheep quality.
18 gauge is the standard gauge for high-end, top quality sinks. If you want to sink to last and not get dents in it, 18 gauge is recommended.
There are some 16 gauge sinks also. Stronger then the 18 gauge ones but usually a perfectly square sink or front apron one. If you like the style of then, enjoy them.
Usually 14 and 16 gauge are used for commercial sinks.