It seems that it is not useful to insert filters in front.
I have a similar hood for the SQ-Ai, and am also trying to gather information on how best to use it. I'm sure they are built to perform the same functions, so perhaps my observations might be useful to you. The slots on the front (side furthest from lens) are designed to hold masks (for longer focal length lenses) or vignetters. There are likely two slots at the rear of the hood (nearest the lens), and are both used for inserting filters. My hood accepts square 75mm x 75mm filters, which originally meant gel filters, not glass. While I'd love to use the gel filters with it, I can't find an affordable secondhand set. I'd be interested in B&W contrast filters and ND filters, but not the graduated ND filters like you were asking after. The hood would not provide the freedom of movement required to properly align the graduated filters to the scene. If I need a GND, I have a Haida 100mm filter holder system I can use.
If you do find a second-hand source for these gel filters, I'm all ears! The vignetters and masks you can probably make yourself out of good quality matte board, like that used in picture framing.
Terry