The type you choose depends on the angle that your flycutter holds the tool; you want clearance back toward the center of rotation so the tool is only cutting on the corner. AL or C will probably work; with A you need the L for clockwise rotation but with C it is reversible.
If your flycutter holds the tool either parallel to the shank or exactly perpendicular you would use GR or GL.
For cutting aluminum use the 370 grade which is for steel; this gives you a tool with a bit of top rake which is needed for aluminum.
When flycutting aluminum on a small machine where you have to keep the speed down because of balance issues I have sometimes found the best surface finish can be obtained by using HSS tools with an extreme top rake; almost so the metal is sliced off in a thin wafer.