test.js

var input = [
  '00100',
  '11100',
  '11000',
  '11011',
  '11011',
];

let gamma = '';
for (let i=0; i<input[0].length; i++) {
  let counter = 0;
  for (let binary of input) {
    if (+binary[i] === 1) {
      counter += 1;
    }
  }
  if (counter > (input[0].length / 2)) {
    gamma += '1';
  } else {
    gamma += '0';
  }
}

console.log(gamma);