Why was the battle of gaugamela so important to Alexander?

In late 331 b.c., Alexander's army headed back north. He turned eastward and invaded Mesopotamia, now ruled by the Persians. Alexander's army smashed Darius's forces at Gaugamela, near the Tigris River. After this victory, Alexander's army took over the rest of the Persian Empire so in cnclusion after this victory he got the rest of persia