Fixative is spirit-based; common modern gesso is an acrylic, water-based (because you can paint oils or anything over acrylic once it's dry). Theoretically the spirit base and water base should not be compatible, so it's best to avoid the fixative under any paint, but I think in practice they would bond through the gesso's high adhesive qualities on the tooth of canvas. But why would you want to? The gesso would obscure any pencil or charcoal drawing. Better to gesso the canvas first, sketch roughly if you must, just to determined the composition, apply fixative to the lines if you must (but I wouldn't - if I did a guiding sketch I'd wipe most of the charcoal off, leaving a "ghost"), then paint with whatever. Painting anything onto a prepared sketch ends up being a kind of "painting by numbers" - stiff and lifeless. That's a sure way to ruin. It's far better to gesso the canvas, broadly sketch the subject with a big brush and thin paint like a wash (whether oils or acrylic etc) or block in the major shapes over the whole canvas in thin, rough tonal values to check on composition (I might use monotone of Burnt Sienna or Ultramarine, seperately or mixed, perhaps with a very little bit of underpainting white or zinc white), and then work over the whole surface with succeeding layers of colour (wiping out with a turpsy or wet rag) to build the image as it emerges. In other words, paint from the inside of the subject out, feeling it, not just the surface, and work "lean to fat". In the same way that adjacent colours affect each other, the volumes, areas, and tonal values do, too, so you need to develop the whole area as one, and let the image talk to you as it grows. Then it's alive, whether it is representational or abstract. Salutations.