If I invent something and don't patent it I assume someone else can? But let's say there is some obvious invention noone ever patented and someone goes to paten

If I invent something and don't patent it I assume someone else can? But let's say there is some obvious invention noone ever patented and someone goes to patent it now even though he obv did not invent it, will he get a patent? Since it is very expensive to patent all over the world is there some way you can do something like the GNU license? So I can say I invented this and anyone can use it however they want and noone can prevent anyone from using it. And that would be free and hold up all over the world?