How much should be the oil pressure for 2000 Chevy Express V8 5.7 L G1500 model?
Does it harm if it is high or low?

Good question. In general there is a oil bypass valve to limit the engine oil pressure to some maximum value, but my '92 has always pegged the original oil pressure guage for anything much above idle. I know that mine isn't the only one that does this, but I don't know why it isn't scaled better...