Hello, again.
I see that the original poster, Emma from Spain, was asking why Maria would be living in Falkirk, and where was her father, James Colvin, throughout the course of her living there and getting married and then going off to Edinburgh with Thomas Gibb.
If you look at the profile for Mary Gibb,
née Colvin, on WikiTree:
https://www.wikitree.com/wiki/Colvin-1639,
her year of birth, 1798, predates her father's marriage to Maria Jackson by four years. Mary appears to have been a natural daughter of James Colvin (1768–1847). There was another natural daughter, Helen, born in 1794, but unlike Helen, for whom there was a baptism record (in the India Office records online at FindMyPast), a similar record has not been found for Mary. In fact, all we have to go by is Mary's repeated assertion (upon marriage, and in her will) that she was the daughter of James Colvin, merchant, of Calcutta. Even so, I believe it is safe to accept this claim.
As to why she was living in Falkirk at the time of her marriage to Thomas Gibb in Falkirk: I would hazard the guess that James Colvin sent his daughter home to Scotland sometime before his 1802 marriage to Maria Jackson. James' brother, George Colvin, had died in Dunipace in 1798, leaving his wife, Margaret Renny. Margaret died at Falkirk in 1838 (though her body was buried at Dunipace), suggesting that she was living in Falkirk after the death of her husband. In fact, Margaret was born at Falkirk; her late father, John Renny (d.1773), had been the bailie of the town. In any event, it seems plausible in the circumstances that a widow, whose husband had had financial difficulties in business and did not leave a will, could use the income that would accompany guardianship of a child (her niece) of a wealthy East India merchant.
Just my 2¢.
cheers,
Alison