Jim's conclusions are summed up on page 81 of the book:

"The Gateacre gateway came from a coffee house that was built by a slave owner [James Bromfield] and frequented by slave owners, but its reputation as a location of slave auctions appears to owe more to association than to historical fact. There is, though, a strong possibility that slaves, and former slaves that had become servants, passed through the gate accompanying their 'masters'. It is a remarkable survivor from 18th century Liverpool, of which far too few remain. It is, like the Bluecoat (funded by a slave merchant) a relic of the town in a period of its most rapid development, a development that benefited greatly from the slave trade".