"GUN" is the number from the door tag (RH, as I recall), not the chassis plate. That's why you are coming up with an erroneous year. That is not officially recorded number and you won't be able to determine th actual VIN/chassis number using it.
-The prefix for an MG chassis plate is "GAN".... and the chassis plate on a '68-'69 would be on the diagonal chassis rail below the carbs. The first digit after the "GAN" will designate what series, followed by the serial number. If that is missing and you want the actual proper chassis number, you'll have to submit a request to British Motor Industry Heritage Trust (BMIHT) in the UK. There should be a left-hand door tag that begins "GBE"; the chassis number can be retrieved with that information.
Posting some photos of the car would help narrow down the year. Please post views with all sides visible.