Kloumann is correct that Janet's parents were Robert Gardener & Janet Crighton (m. July 1781 in Kimoul).
What is your evidence for that?
If that 1841 census is the right family, Janet is listed as aged 45. Adults' ages in 1841 were supposed to be rounded down to the nearest 5 years, so if 45 is accurate she could in fact have been any age between 45 and 49. The 1841 census was taken on 7 June, so she would have been born between 8 June 1791 and 7 June 1796.
The daughter of Robert Gardner and Janet Crighton would have been 53 on census day 1841, and should have been listed as aged 50 (if at all, of course).
There is a Janet Gardiner, daughter of David Gardiner, baptised in Abernyte in 1793, and a dozen or so Jan(n)et Gard(i)ners born further away between 1791 and 1796, so you need to have some positive proof of your Janet's parentage. Especially as Duncan and Janet named their second son David and did not name a son Robert.
must be a record somewhere Duncan's birth record ...
Not necessarily. There are lots of people whose baptism records, if they ever existed, have not survived.
