i'm not sure what you are trying to accomplish. what you call a "sash"
is really a jamb (a sash is a part of the window that holds the glass).
this gap between the drywall and the jamb is usually covered with
trim. the only time it is not is when drywall wraps around and butts
into the window, but that involves adding peices of drywall, applying
a corner bead, and then three coats of mud followed by sanding and
painting. on your door openings that have wood jambs and wood casings,
you can just wrap them with drywall. or perhaps you just want new
trim?