1.
serious match; genuine contest; real fight
A match or contest fought with full effort and no pre-arrangement, originally from sumo wrestling.
2.
for real; serious; genuine; all-out
Used more broadly to describe any situation or effort that is completely serious and genuine, without pretense.
Originates from sumo wrestling, where it describes a bout fought with genuine effort rather than being fixed (八百長). The name is said to come from the sound of wrestlers' heads clashing (ガチンと). In modern slang, it has expanded far beyond sumo to mean anything done seriously and for real. Often shortened to ガチ in casual speech.