I have managed to answer my own question - through a little digging -

Using the DELETED table as the before data.

New Trigger;

ALTER TRIGGER [dbo].[trgGenMasterUpdate]

ON [dbo].[GenMaster]

AFTER UPDATE

AS

BEGIN

SET NOCOUNT ON;

insert into GenMasterUpdates (GlCode, Action, Before, After )

select INSERTED.GlCode, 'UPDATE', DELETED.Description, INSERTED.Description

from INSERTED join GenMaster on INSERTED.GlCode = GenMaster.GlCode

join DELETED on INSERTED.GlCode = DELETED.GlCode

END