John Barnes says Gillingham have the ability to challenge the top teams in League One, if they keep the attitude that got them promoted last season.
The former Liverpool and England star is now in charge of one of the Gills rivals, Tranmere Rovers.
Barnes, 45, told BBC Radio Kent: "If they believe they can, then they can.
"They're a fair size club and I suppose if you look at what they've achieved in the past, they probably shouldn't have been in the division they were."
He added: "Now they're promoted it depends on their own belief."
Barnes won 79 England caps and made over 500 appearances for Watford, Liverpool, Newcastle United and Charlton Athletic.
He began his coaching career at Celtic, but left after only 10 months of an unsuccessful spell in charge.
He took over at Prenton Park in June, ending a nine-year exile from club management, after a spell in charge of the Jamaican national team.
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