You know I can't resist a challenge, don't you, Cloverjo? ;-)
Using my original dates as a guide once again . . .
Flights from Bristol to Glasgow (with easyJet) are available as follows:
Dep 0715, Arr 0830, £54.49
Dep 1425, Arr 1540, £30.49
Dep 1615, Arr 1730, £24.49
Dep 2000, Arr 2115, £24.99
Assuming that the 1615 flight was to be taken, travel from Chepstow to the airport could be by train to Newport and then by National Express to the airport. (Advance train ticket £7.30, leaving Chepstow at 1251, arriving Newport 1311. Coach £13.00, leaving Newport at 1335, arriving at the airport 1430). Alternatively, pre-booked car parking at Bristol Airport would cost £38.99.
A return ticket on the Glasgow Airport Express bus, into the city centre, would cost £9.50. (They run every 10 minutes, with a 15 minute journey time).
Then the 1841 train from Glasgow Queen Street gets to Gleneagles at 1925. (£7.60 Advance fare).
_____________________________________________________
Coming back, there are flights from Glasgow to Bristol as follows:
Dep 1425, Arr 1535, £30.49
Dep 1755, Arr 1905, £32.49
Dep 2140, Arr 2250, £26.49
The last flight might be acceptable if Jubieanna has left a car at Bristol Airport but I'll assume that public transport is to be used to get back to Chepstow and so work on the assumption of taking the 1755 flight from Glasgow.
There seems to be a big gap in rail services from Gleneagles to Glasgow, so it would be necessary to catch the 1253 train, arriving Glasgow QS at 1348. (Advance fare £7.60). That's 4 hours before the flight, so a leisurely lunch could be taken in Glasgow before catching the airport bus.
On arrival in Bristol, there's a 2015 National Express service to Newport, arriving 2105. (£13.00)
Then the 2127 train gets into Chepstow at 2149 (£7.30)
So (assuming the use of public transport to and from Bristol Airport), that's £7.30 + £13.00 + £24.49 + £9.50 + £7.60 + £7.60 + £32.49 + £13.00 + £7.30 = £122.28, which isn't that cheap but it's still a lot less than the standard rail fare and somewhat quicker. (If Jubieanna can get a friend to drive her to Bristol Airport, and then pick her up when she comes back, it would probably cost under £20 for the fuel for the two return trips, saving around £20).
Links:
http://www.easyjet.com
https://www.traveline.cymru
http://www.nationalrail.co.uk/
http://www.nationalexpress.com/home.aspx
http://www.bristolairport.co.uk/parking/long-stay-car-park
https://www.firstgroup.com/greater-glasgow/routes-and-maps/glasgow-airport-express