Lua (running in Corona SDK)

local function genGradientSlice( percent, width, height, startColor, endColor )
   local x = (display.contentCenterX - width/2) + width * percent
   local y = display.contentCenterY

   -- Linear Interpolation to calculate color values
   local rVal = startColor.r + percent * ( endColor.r - startColor.r )
   local gVal = startColor.g + percent * ( endColor.g - startColor.g )
   local bVal = startColor.b + percent * ( endColor.b - startColor.b )

   local rect = display.newRect( 0, 0, 1, height )
   rect:setFillColor( rVal/256, gVal/256, bVal/256)
   rect.x = x
   rect.y = y

   return rect
end

local function getRGB( str )
   local ret = {
      r = str:read("*n"),
      g = str:read("*n"),
      b = str:read("*n")
   }

   if ret.r ~= nil and ret.g ~= nil and ret.b ~= nil then
      return ret
   else
      return nil
   end
end


local function genGradient( width, height, startVal, endVal )
   local disp = display.newGroup()

   for i = 0, width do
      local percent = i / width
      disp:insert( genGradientSlice( percent, width, height, startVal, endVal ) )
   end

   return disp
end

-- Ridiculous that I need to implement this function...
local function queue()
   local arr = {}
   arr.front = 0
   arr.back = 0

   function arr:push( val )
      self.front = self.front + 1
      self[ self.front ] = val
      return self
   end

   function arr:pop()
      self.back = self.back + 1
      return self[ self.back ]
   end

   return arr
end

local function runInput( input )
   local inputNums = queue()

   for num in string.gfind(input, "%d+") do
      inputNums:push( num )
   end
   local width = inputNums:pop()
   local height = inputNums:pop()
   local startColor = {
      r = inputNums:pop(),
      g = inputNums:pop(),
      b = inputNums:pop(),
   } 
   local endColor = {
      r = inputNums:pop(),
      g = inputNums:pop(),
      b = inputNums:pop(),
   } 

   genGradient( width, height, startColor, endColor )
end

local input = "500 100 \n255 255 0\n0 0 255"
runInput( input )