"The EU is a much more effective partner... Compared to 20 years ago when fewer members were not able to come up with a common view," said Gardner at a press conference in Brussels.
Gardner, who took over the post last month, said the response to Russia's intervention in Ukraine -- including sanctions and the signing of an association agreement with Kiev -- had been impressive.
"I'm conscious it is 28 member states, not just one, with different historical and cultural backgrounds and a dependency on Russian gas. But I'm impressed with the speed with which they have acted and the close cooperation," he said.
He also raised the question of energy security. Some European lawmakers have pressed for energy to be included in the ongoing negotiations on an EU-US free-trade agreement known as the Transatlantic Trade and Investment Partnership (TTIP).
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Gardner said this was "not necessary".
"We do a lot outside of the TTIP to address these issues. It doesn't need to be part of this agreement."
On security, he recalled statements made by Barack Obama last week in Brussels, where the US president said "freedom is not free" and called on NATO members to meet their spending obligations on defence.
"Our ability to be active in those countries is not high. We see it as complementary, not duplicatory," said Gardner.
He added that he was worried about "some of the narratives gaining ground" around the TTIP negotiations, particularly in relation to deregulation and the creation of appeals courts to protect investors, which the United States has been championing.
"I will be on the front foot, in the European Parliament and the press, to show how these arguments are not grounded in fact," he said.