If gst number is showing invalid but it is valid than need to update regular expression of the field to ^[0-9]{2}[A-Z]{4}[0-9A-Z]{1}[0-9]{4}[A-Z]{1}[1-9A-Z]{1}[0-9A-Z]{2}$
for that right click on that field or f4 then click on the edit button of regular expression  and update the regular expression with the expression mentioned above