Mr Gill is expected to soon give up his role as a member of the European Parliament after his success in Wales.
He claimed Ukip was the “Leicester City of politics”, adding: “We were written off five years ago and look how well we are doing now.”
Mr Hamilton said: “UKIP has made a great breakthrough today.
“I don’t think the Assembly will ever be the same again.”
A delighted Mr Reckless vowed Ukip “will be a breath of fresh air in the Assembly”, adding: “It’s been too cosy for too long.”
Mr Hamilton (Mid and West Wales), Mr Gill and Michelle Brown (both North Wales), Mr Reckless and David Rowlands (both South Wales East) and Caroline Jones (South Wales West) are among the Ukip candidates to have confirmed seats in the Welsh Assembly.