Key US ally Britain is expected to slash 10% from its 36.9 billion pounds (42.2 billion euro) budget, while experts at Germany's Defence Ministry have listed potential savings of more than 9.3 billion euros ($13 billion).
"It is a concern," Gates told reporters on a flight to Brussels. "My worry is that the more our allies cut their capabilities, the more people will look to the United States to cover whatever gaps are created."
"And at a time when we're facing stringencies of our own, that's a concern for me."
NATO defence and foreign ministers gathering in Brussels will discuss ways to cut costs, particularly in overheads, including by reducing the number of NATO agencies.
Gates said he wanted European allies to do what he aimed to do in Washington: reinvest cost-savings in critical capabilities. He cited missile defence and efforts to counter roadside bombs, the number-one killer of foreign troops in Afghanistan.
"Any savings realised as a result of efficiencies should be ploughed back into these critical capabilities," Gates said.
NATO Secretary-General Anders Fogh Rasmussen wants NATO allies to agree at a summit in Lisbon next month to invest 200 million euros over 10 years to link their existing missile defence capabilities and missile interceptors Washington plans to deploy in Europe.
Gates said he believed there was broad support for the scheme in Europe, but some allies such as France say they want more details of the proposal, including who would command the system and what rules of engagement it would be covered by.
France also wants to discuss interoperability of the systems.
However, a European NATO diplomat said there was "a reasonable hope for an agreement on the system at Lisbon".
NATO has also called on Russia, the alliance's old Cold War foe, to cooperate in missile defence, but Russia has remained cautious and is yet to respond to an invitation to join the Lisbon summit.
NATO says the system is aimed at protecting the territory of NATO states from an increased threat from countries such as Iran, and is not aimed at negating Russian power.
Gates said there was very little additional expenditure European nations needed to make, beyond what they had already committed.
"The cost in going forward with this over and above what has already been approved by the alliance is really very modest," Gates said. "But you never know whether you have 28 votes - until you vote."
(EurActiv with Reuters.)