What did we gain from winning world war II?

at the end of WW2 the USA was the Richest Country on the Planet and its allies had Not started to pay back lend lease

Operation Paperclip Got Nazi Scientists for the USA including the Rocket scientists

Main gain was the creation of the UNO that provided for a platform to discuss issues before the start of any outright war. World nations thereby got an opportunity of expressing their views before taking sides. World public opinion changed in favour of democracy and against totalitarian forms of government. In spite of the cold war, there was a shift in favour of coexistence of different systems.

Well, we can still cuss at politicians, and it's safe in the US to be a Jew, a Jehovah's Witness, a Gypsy or any number of other things.

stopped Hitler from taking over Europe

Peace

what a great question - what did we gain - we lost - 72 million dead - I can see no gains - but a few people gained billions in their bank account

Well, what White Americans gained from winning World War Two was a successful stand against their own racial prejudice. World War Two was a big turning point for Blacks and led inevitably to a different attitude, surprising even many White World War Two veterans in its ramifications. And Jews who had not had access to many hotels and resorts and country clubs in America broke that social barrier after the Holocaust: There is a movie about that starring Gregory Peck called "Gentlemen's Agreement". Mainly the gain was social and moral rather than physical. It was White Americans transforming themselves. It could have gone the opposite way: Too often forgotten is the German Bund groups marching in Manhattan wearing swastika armbands in the 1930s, holding rallies at Montauk and Yaphank, Long Island, trying to influence Congress to go into the war on Hitler's side. If the US had JOINED Hitler he would probably have won and the world would be very different than it is today. By siding AGAINIST Hitler, White Americans chose inevitably the opposite path, though they did not all realsie that it would lead inevitably to Civil Rights.

For a start - who is "we", that won WW2? Are you British, Russian, Australian, French, American, other?

The allies ended the push by Germany and Japan to take control of the world.

Japan wanted the ability to have access to the world markets and resources- they got that after WWII
Germany wanted its war debt forgiven and access to the world market for its products and ability to purchase materials on the world market- they got that after WWII
The US taxpayer paid to rebuild Japan, Germany, France and England.
As a result the US lost jobs and has ended up footing the bill for all the little wars ever sense.
So when the Allies won WWII the US lost the war by winning the little battles along the way.