GB18030 has no 3-byte sequences. Only 1, 2, and 4. There are more important qualities than optimizing byte length. Processing GB18030 as Unicode scalar values

GB18030 has no 3-byte sequences. Only 1, 2, and 4. There are more important qualities than optimizing byte length. Processing GB18030 as Unicode scalar values involves lookup tables. A single byte error cascades potentially further than in UTF-8. If optimizing Chinese byte length is really important for you, UTF-16 is easier to work with than GB18030.