Temporarily set the object's ScaleMode property to vbPixels so you can easily draw each pixel exactly once. Then use a loop to draw lines in different colors starting at the left edge of the object and moving to the right by one pixel each time.

' Shade the object from left to right.
Private Sub ShadeObject(ByVal pic As Object, _
  ByVal r1 As Byte, ByVal g1 As Byte, ByVal b1 As Byte, _
  ByVal r2 As Byte, ByVal g2 As Byte, ByVal b2 As Byte)
Dim old_scalemode As Integer
Dim wid As Integer
Dim hgt As Integer
Dim i As Integer
Dim dr As Single
Dim dg As Single
Dim db As Single

    old_scalemode = pic.ScaleMode
    pic.AutoRedraw = True
    pic.ScaleMode = vbPixels
    wid = pic.ScaleWidth
    hgt = pic.ScaleHeight

    dr = (CSng(r2) - r1) / wid
    dg = (CSng(g2) - g1) / wid
    db = (CSng(b2) - b1) / wid

    For i = 1 To wid
        pic.Line (i, 0)-Step(0, hgt), _
            RGB(r1 + i * dr, g1 + i * dg, b1 + i * db)
    Next i

    pic.ScaleMode = old_scalemode
End Sub