When a player turns a gig down, their chair is open. You get an email, and the gig page shows Open chairs, and who could cover them.
GLAR suggests roster players who play that part (as their main instrument or under Can also play) and haven't answered yet.
The offer names the sub's own Usual chair from the roster. If that's not the chair you mean, say so in a quick email.
If the player who said no changes their mind, you get an email saying they are back in, with any other open chair they could take.