Thanks for your help! I can see on those records where it says Kirkmichael and Garrel were united in the 1860's, and I also used the Fasti to search for the name Porteous, just in case it was there, but no luck.
Regarding the precise whereabouts of Kirkmichael church, I tried googling stuff again and discovered that it is 6 and a half miles NW of Lochmaben. And Garrel is 3 miles from Kirkmichael Church, though it says 3J miles, and the letter j must be a misprint. Anyway, it seems that Garrel Church is a ruin and has been since before the 1800's, so this rules out Garrel church as being the source of any birth records for James & Janet Porteous. It must be Kirkmichael Church.
I googled "Old photo of Garrel church, Dumfriesshire" and saw sites on the list, both for Garrel and Kirkmichael. Some good images of the churches and interesting photos showing what the area looks like.
I think I will send to the Dumfries & Galloway F.H.S. for some booklets they have, but there is another booklet that they don't have, which is for Births at Kirkmichael, 1727 - 1819. I have still to find a site that would sell me it, but it looks like my best bet.