If you manually set your default shape to have a gradient fill then the following will work. It appears the object model is broken in as much as setting TwoColorGradient does not alter the Solid setting and makes calls to gradientstop properties fail.

Sub EDIT_ITEM()
    
    Dim shpTemp As Shape
    
ActiveSheet.Shapes("item").Select
    Selection.Copy
    Range("F15").Select
    ActiveSheet.Paste
    Selection.Name = Range("E5").Value & Range("F5").Text
    Set shpTemp = ActiveSheet.Shapes(Range("E5").Value & Range("F5").Text)
    With shpTemp.Fill
        .Visible = msoTrue
        .TwoColorGradient msoGradientHorizontal, 1
        With .GradientStops(1)
            .Color.RGB = RGB(Split(Range("G5"), ",")(0), Split(Range("G5"), ",")(1), Split(Range("G5"), ",")(2))
            .Position = 0
        End With
        With .GradientStops(2)
            .Color.RGB = RGB(Split(Range("H5"), ",")(0), Split(Range("H5"), ",")(1), Split(Range("H5"), ",")(2))
            .Position = 1
        End With
    End With

End Sub
you can set RGB directly
        .Fill.ForeColor.RGB = RGB(0, 255, 0)