Complex sentence: Around the 3 rd century they were assimilated into the Tiele (鐵勒, also named Gaoche (高車) or Chile (敕勒)), who gradually expanded westward into Central Asia.
Split into simpler sentences: Around the 3 rd century they were assimilated into the Gaoche (高車) or Chile (敕勒). From these emerged the Tiele (鐵勒) who gradually expanded westward into Central Asia.