World War 1
Greece (Gained land from the former Ottoman Empire 1920 and Bulgaria 1923)
Soviet Union (loss of lands to various nations)
Czechoslovakia (Formed out of the former Austrian-Hungarian Empire – 1919 through formal treaty)
Poland (Declares independence from the Russian Empire and gains additional land from Austria-Hungry 1918-1919)
Germany (loss of lands to various nations)
Italy (Gained pieces from the former Austrian-Hungarian Empire and newly formed Yugoslavia 1918-1920)
Estonia (Declares independence from the Russian Empire 1918)
World War 2
Greece (Gains land from Italy 1945)
Soviet Union (Gains various territories such as parts of Poland, the Baltic States and others 1939-1945)
Czechoslovakia (Becomes annexed by Germany in 1938 and then regains independence. Losses additional territory in 1945 to the Soviet Union)
Poland (Becomes annexed by Germany and the Soviet Union in 1939. Regains independence but does not regain land lost to the Soviet Union, but does gain additional land from Germany)
Germany (Losses pieces of land to various nations such as parts of land lost to Poland 1945. Though it gains and losses land through the war)
Italy (cedes land to Greece and Yugoslavia 1945)
Estonia (losses independence when the Soviet Union annexes it in 1939)
For both World War 1 and 2 Sweden was neutral and I am not aware of them either losing or gaining territory
Soviet Union (then known as Russia).
Estonia (new country established as a result of WW1).
Poland (new country established as a result of WW1).
Czechoslovakia (new country established as a result of WW1).
Estonia (disappeared as an independent country, became part of the Soviet Union).