I Think Thameslink may once again service Guildford when completed. It did for a while some years ago.
It won't. The current planned list of southern destinations is Brighton, Three Bridges, Horsham, East Grinstead, Caterham, Tattenham Corner, Tunbridge Wells, Ashford, Maidstone East, Sevenoaks and Bellingham. Although it does seem to change every now and again.
Guildford was the southern terminus for St Albans stoppers from about 1989 to 1993 (before that it was Sevenoaks via Catford, as today). It was switched to the Sutton loop in anticipation of privatization, because they didn't want Thameslink interfering with two franchises at once (South Western and South Central).