GPU Miners are like Gaming Rigs with many cards. They generate around ~400 MH each depending on strength. Only takes a few AntMiners to disrupt a majority non-ASIC. Network TH>GH>MH>KH
PoW requires very strong miners to be powerful enough to process a block. A single outdated miner is too weak to break even in rewards. Thus a pool of Antminers are required for profit