Striker Patrick Agyemang has already agreed a permanent switch to Portsmouth having spent the second half of the season on loan at Fratton Park.
And Boro boss Westley has also released defenders Andy Iro, Miguel Comminges and Bondz N'Gala at the end of their contracts.
Full-back Gray, midfielder Grant and Canada striker Haber, who are entering the final year of their deals, have been transfer listed.
Chairman Phil Wallace told the club website:"Our income for last season was down 30 per cent and this impacts on the maximum permitted budget we can allocate for players next season.
"Graham wants to bring new lads in to replace those listed as he doesn't feel they'll get many game minutes under him next season.
"If that's the case it's better for all that they move on to develop their careers. We've had some interest already so we'll see what will happen."