Just uploaded a new version of GSTool. It supports some settings that don't work in the current version but will in future versions, most notably, a new object type: "Background Image". An object set to this (one per graphic set) will be used as a background image on any level that uses the graphic set in question as its primary graphic set.
Most other newly-supported features are for long-term purposes, not immediate future.
Note that colors work a bit differently with this new version. You no longer set particle colors; instead, you can set the mask color (used eg. for bridge pixels), minimap color, and background color; with the remaining colors being unused (but the first three have planned future use; GSTool, when opening an old DAT graphic set, will automatically set the colors correctly for the new uses, including these future ones. I'm not sure if it sets them correctly when opening an old graphic set from any other format.)