Its best practice to have TOP 1 1 always.
What if I use SELECT 1 -> If condition matches more than one record then your query will fetch all the columns records and returns 1.
What if I use SELECT TOP 1 1 -> If condition matches more than one record also, it will just fetch the existence of any row (with a self 1-valued column) and returns 1.
IF EXISTS (SELECT TOP 1 1 FROM tblGLUserAccess WHERE GLUserName ='xxxxxxxx')
BEGIN
SELECT 1
END
ELSE
BEGIN
SELECT 2
END