Fifteen GLA officers carried out 16 separate inspections uncovered Indian, Lithuanian and Polish workers picking and packing cabbages, cauliflowers, leeks and spring onions, or packing salads and preparing plants in nurseries.
Workers are alleged to have been paid below the minimum wage, offered no holiday or sick pay, and no contracts with terms and conditions. They were transported in an untaxed van then left in a field without supervision and transport from 6am to 4pm, the GLA said.
A GLA spokesman added: "We can't identify companies until investigations are complete. But nobody hates a rogue gangmaster as much as a legitimate one."
Earlier surprise inspections under the GLA's Operation Ajax found gangmasters acting legally, but the second wave of swoops met with several "totally unacceptable" practices.
GLA chairman Paul Whitehouse said Lincolnshire had its problems with rogue gangmasters. Four gangmasters in the county have had their licences revoked since April.
"The good gangmasters and the businesses that use them have shown if we work together we can protect the workers and stop those who make profits illegally," he said.