Try setting the worksheet of the cells in the Range.

Maybe

Set gml = Sheets("Input").Range(Cells(1, 13), Cells(5, 15))

Mike

Dim G as long
dim gml as range

with sheets("input")
 for g = 1 to 5
    set gml = .range(.cells(g,13), .cells(g,15))
    'do something with gml here???
 next g
end with

or
 for g = 1 to 5
    set gml = sheets("sheet1").cells(g,13).resize(1,3)
    'do something with gml here???
 next g

Notice the placement of the dots in front of .range and .cells
Not quite sure what you want here. As shown, you get 1 in all 15 cells.
Changing the comment to the next to last line gets you only 3 on row 5

Sub doit()
For g = 1 To 5
With Sheets("Input")
Set gml = .Range(.Cells(g, 13), .Cells(g, 15))
gml.Value = 1
End With
Next
'gml.Value = 1
End Sub