Beans become a complete protein when mixed with other foods (ie red beans & rice). Beans are a great addition to many foods/dishes like: salsa + rice, salads, soups, etc....
I love: Lentils, Black Beans, Light/Dark Kidney beans, etc etc....
As for the gas, either take 1) Beano or 2) you're body will get used to it eventually & the gas will be reduced or 3) when cooking a soup, etc with beans add a tiny bit of baking soda to it -- it helps cut down on the gas -- but just a little -- too much will make the dish taste funny.
Protein lacking aminoacids... It goes like this: Protein is made up of aminoacids: Some of these aminoacids your body can make (non-essential Amino) and some of them your body cannot make (essential Aminos) so you have to eat them to get them...
Incomplete protein is protein lacking some essential aminos, so basically your body gets protein, but it is not full of all the aminos it needs to build new tissue itself. Since it cannot complete the aminos in the protein (because it cannot make them without breaking fown your own muscle tissue for aminos), well, you get the picture: You eat the incomplete protein, it will fill you up, but it will not help you building muscle.