how can create dynamic gradient?

Here's a function you can use to generate vertical and horizontal linear gradients. Keep in mind, this function takes over 1000ms on my streaming stick 4k for a 1920x1080 image, so use it sparingly. 

function GenerateGradient(width as integer, height as integer, startRgba as longinteger, endRgba as longinteger, lineDirection = "vertical")
  bitmap = CreateObject("roBitmap", { width: width, height: height, alphaEnable: true })

  'draw vertical lines from left to right
  if lineDirection = "vertical" then
    numSteps# = width
    'draw horizontal lines from top to bottom
  else
    numSteps# = height
  end if

  ' separate rgba values to compute the step between each gradient band
  red# = (startRgba and &hFF000000&) >> 24
  redEnd# = (endRgba and &hFF000000&) >> 24
  redStep# = (redEnd# - red#) / numSteps#

  blue# = (startRgba and &h00FF0000&) >> 16
  blueEnd# = (endRgba and &h00FF0000&) >> 16
  blueStep# = (blueEnd# - blue#) / numSteps#

  green# = (startRgba and &h0000FF00&) >> 8
  greenEnd# = (endRgba and &h0000FF00&) >> 8
  greenStep# = (greenEnd# - green#) / numSteps#

  alpha# = startRgba and &h000000FF&
  alphaEnd# = endRgba and &h000000FF&
  alphaStep# = (alphaEnd# - alpha#) / numSteps#

  for i = 0 to numSteps#
    red# += redStep#
    blue# += blueStep#
    green# += greenStep#
    alpha# += alphaStep#

    'merge rgb into single int
    color& = (Int(red#) << 24) + (Int(blue#) << 16) + (Int(green#) << 😎 + (Int(alpha#))

    if lineDirection = "vertical" then
      'draw a line at (i, 0) for the full height of the image
      bitmap.DrawRect(i, 0, 1, height, color&)
    else
      'draw a line at (0, i) for the full width of the image
      bitmap.DrawRect(0, i, width, 1, color&)
    end if
  end for

  bitmap.Finish()

  png = bitmap.GetPng(0, 0, width, height)
  uri = "tmp:/linear-gradient-" + str(width) + "x" + str(height) + "-0x" + stri(startRgba, 16) + "-0x" + stri(endRgba, 16) + "-" + lineDirection + ".png"
  png.WriteFile(uri)
  return uri
end function

And you can call it like this:

verticalGradientUrl = GenerateGradient(1920, 1080, &h00000000, &hFF0000FF, "vertical")
horizontalGradientUrl = GenerateGradient(1920, 1080, &h00000000, &hFF0000FF, "horizontal")

The function has not been optimized, meaning it will draw a line for every pixel in the gradient, even if several lines would have the exact same color. It could be optimized to draw a rectangle that is as big as the height/width of the color band. 

If you're in control of the poster size, then you should consider generating a gradient that's 1 pixel in the opposing direction. (i.e. 1920x1 or 1x1080), then you can stretch the poster to the size you need in that opposing direction.