While ArtRage 2.5 doesn't currently have a fill tool, there are a couple of things you can do to help.
You can convert your layer to a stencil, set the stencil to Ruler mode, and that will prevent your tool from going outside the area as you paint in the area you're filling.
To have a smooth gradient you can overlay a gradient stencil, which will affect the opacity of the paint as you apply it.
ArtRage 3 will have a flood-fill tool, although we dont currently have plans to support gradient fills.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"