Just weeks before the British leader is expected to start the formal process of Brexit, Gentiloni spoke in conciliatory terms about a divorce that has already sparked harsh words on both sides.
"We are aware of the fact that the negotiations will not be easy," Gentiloni said at a joint press conference at May's Downing Street office.
"We also know, and this will be certainly the Italian attitude, that we need to show a constructive and friendly approach.
"There is absolutely no point in having a destructive negotiation between the EU and the UK."
The two leaders discussed the issue of the post-Brexit status of EU citizens living in Britain, alongside the question of migration into Europe, counter-terrorism and the challenge of Russia.
May has drawn criticism for refusing to guarantee the rights of Europeans in Britain, but has argued that she must first ensure reciprocal assurances for Britons living elsewhere in the EU.
She said: "When I trigger Article 50 I will make clear that I want this issue to be addressed at an early stage of the negotiations."
May has said she wants to activate Article 50 of the EU's Lisbon treaty, which starts two years of exit negotiations, by the end of March.
British MPs on Wednesday voted overwhelmingly to support a bill empowering May to begin the Brexit process, and the legislation will now move to the unelected upper House of Lords.
But MPs rejected an attempt to amend the bill to include a guarantee for EU citizens.
Gentiloni said: "We have to reassure our citizens, Italians living in Britain and Britons living in Italy, that their rights will be respected and there will be what we call a reciprocal treatment."