s.rb

RANGE_MIN = 360
RANGE_MAX = 1140

def fill_gaps(ranges)
  gaps = []
  last_rng_end = RANGE_MIN

  ranges.each do |rng|
    if rng[0] > last_rng_end
      gaps << [last_rng_end, rng[0]]
      last_rng_end = rng[1]
    end
  end

  if last_rng_end < RANGE_MAX
    gaps << [last_rng_end, RANGE_MAX]
  end

  gaps
end

puts fill_gaps([[480, 720], [780, 840], [1020, 1080]]).inspect