He said he supports devolution but is "not enthusiastic" about further powers coming to Wales.
Speaking on BBC's The Wales Report programme, Mr Gill said the assembly's current powers have been "badly used".
He also said local parties will decide who represents UKIP in May's Welsh assembly election.
Mr Gill was responding to suggestions that former  Tory MPs Neil Hamilton and Mark Reckless were set to be imposed as candidates by the party centrally.
"As a principle, I think that it's madness to have people standing who the local party don't want to stand for them.
"Which is why the local party will now decide who represents them," he said.
He said the upcoming elections give the party a chance to show they can be disciplined and do "good things for the people of Wales".
"We want to be a constructive part of the assembly," he added.
"We want to be there to get the voices and the views of those people who vote for us heard and we need to make sure that people start to believe in this institution for the right reasons."
UKIP wants to be a "constructive part of the assembly" and can do "good things", Welsh leader Nathan Gill has said.