Although Gladys Z. Click and James Graham Duff were married August 3, 1920 their courtship, in whatever form it may have taken up to that point, apparently lasted the better part of the decade beforehand as this 1913 postcard would attest to. Seeing as James Duff moved to America in 1906 it's reasonable to assume that he became acquainted with Gladys or the Click family much sooner than 1913.
The writer, I'm assuming it's Kate Duff as there are a few others from her in the collection, seems well-acquainted with Gladys and at this point the two have likely met and spent time together either in Scotland or Detroit.
As with most of the received cards Kate mentions "us" and it's quite possible that James Duff made solo trips to his homeland though, just as well, it could be her husband, if she had one, or various other possibilities. Either way it establishes the fact that Gladys Click and the Duffs were well-acquainted by this point.
As for their marriage: as mentioned previously it took place on August 3, 1920 in Highland Park, Michigan. Minister D. B. Davidson officiated ad Grace B. Click and James C. Ferriss were witnesses.
The marriage registry ceases to be fully helpful after that information is revealed due to the blacked-out section in the middle which alludes to the bride and groom's fathers and mother's maiden names. James's mother looks to be named Catherine Graham and Gladys's is listed as Connie Coulan though I believe her name was Caroline.
I also came across an immigration manifest for their 1923 trip to Scotland. Although only James is shown on the record multiple pictures from July 1923 show both Gladys and he in various locations around Scotland. The manifest shows that he traveled on the ship Montlaurier and that it departed from Liverpool and Greenock and arrived in Quebec, Canada on August 4, 1923.