Children are not permitted to play the category C machines, which have a maximum stake of £1 ($1.29) and a maximum prize pay-out of £100.
A GC review across 170 pubs found 84% are failing to prevent under-18s from accessing the machines.
According to the GC, pub staff are responsible for ensuring children do not use them, while clear signage should indicate age restrictions.
However, a similar review last year found 88% of pubs in England failed to prevent children accessing fruit machines, so there has been a marginal improvement across the board.
Helen Rhodes, GC Programme Director, said: "The pub industry must accelerate action to enforce these rules.
"Pubs must take age verification on machines as seriously as they do for alcohol sales; and they risk losing their entitlement to offer machines if they do not.
"We urge the pub sector to respond to this opportunity to protect children and young people, and to prevent the need for local authorities to take enforcement action."