The gain line is a term used in rugby union

Rugby union

Rugby union, often simply referred to as rugby, is a full contact team sport which originated in England in the early 19th century. One of the two codes of rugby football, it is based on running with the ball in hand...

. It is an imaginary line [parallel to the halfway line] drawn across the pitch at the point where there is a breakdown in open play, such as a ruck, maul

Maul

A splitting maul also known as a block buster, or block splitter is a heavy, long-handled hammer used for splitting a piece of wood along its grain. One side of its head is like a sledgehammer, and the other side is like an axe.- Wedged mauls :...

 or scrum

Scrum (rugby union)

In rugby union a scrum is a means of restarting play after a minor infringement. It involves up to eight players from each team, known as the pack or forward pack, binding together in three rows and interlocking with the free opposing teams forwards. At this point the ball is fed into the gap...

. Advancing across the gain line represents a gain in territory. Playing "beyond the gain line" is a common phrase coaches use to teach their players the fundamental goal of rugby (gaining space while maintaining possession of the ball).