His comments come despite incoming US President Donald Trump championing Brexit, saying in an interview at the weekend with the Times newspaper that Britain was "right" to exit the EU.
Gardner, who leaves office this month after a three-year stint in Brussels, said he will "not be shy" about speaking out about the potential risks the UK faces by voting to leave the EU.
He listed some of the achievements he believes the EU and US had achieved during his time as ambassador, including data exchange legislation agreed with Parliament, in which he took "great satisfaction."
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Countering terrorism and furthering the digital single market were also noteworthy achievements, he said.
"We have made significant strides and I just hope that in four years we are not back to where we started," he said. "What we must not do is act as a cheerleader for Brexit."
In a robust defence of both the EU and NATO, he said, "I hope the new team in America will appreciate that Europe is a provider of security."
Looking back on his term in office, he said, "We have done some good things, including restoring trust."
Gardner, who is taking up a short-term teaching post the College of Europe at Bruges, was particularly scornful of EU critics, saying he wanted to "go out in a ball of fire".
"I now feel unshackled and will not be shy in speaking out. A lot is at stake so this is needed more than ever."
He said, "I cannot hide my sense of concern about the temptation for the new team to deal bilaterally with EU countries with the perception that it should deal only with its 'best friends' and ignore the others.
"This approach should be avoided at all costs."
Germany and France, he said, would now assume a leadership role in defending the EU.
With populism on the rise and the popularity of right-wing politicians like Front National leader Marine Le Pen, Gardner said, "Never before have they - Germany and France - been asked to carry the flame of democracy. My hope is that they will do this.
"The problem is that nothing can be taken for granted now. I have been profoundly shocked by the vulgarity, violence and xenophobic nature of our political discourse."
In a reference to the "shocking caricature" of the EU by Nigel Farage and others, he said, "We seem to have lost our sense of outrage. Five years ago, this sort of thing would have been off the political reservation but now we just go 'oh um.'"
In an interview with reporters, Gardner also attacked Russian "meddling" in the U.S electoral process, saying, "It is acknowledged that Russia interfered and meddled in the US elections and, in light of this, it is inconceivable and even shameful to consider lightening sanctions on Russia."
This comes despite Trump saying at the weekend that, as President, he may consider lifting sanctions on Moscow if Russian President Vladimir Putin cooperates in tackling Islamic State.
Turning to the future, Gardner urged the EU not to cede its "leadership" role on climate change, adding, "Who would have thought a few years back that China now would be the sponsor of measures to cut global greenhouse emissions?"