Irregular marriages were abolished in Scotland in about 1940. So any marriage in Gretna Green in the 1960s should be in the normal statutory records on Scotlandspeople.
Sorry you are (partially) wrong - The
Marriage (Scotland) Act 1939 is the legislation you refer to. Although it did make major changes by abolishing "Marriage by Declaration" it did NOT abolish all forms of irregular marriage (one escaped). Marriage by declaration was the form most popular in irregular marriages in areas like Gretna where they appear to have kept fairly good business records of all transcations.
If the marriage took place in 1962 I would agree with you, it was either a "regular" marriage in which case it should be on Scotlands People or it was an informal ceremony (not a marriage) and may not be recorded.
Irregular marriage in the form of "Co-habitation & Repute" continued to be recognised as a form of marriage until The
Family Law (Scotland) Act 2006 abolished the recognition of such unions formed after May of that year as marriages.