From the book:
p37: "Gilsland was briefly a fashionable spa during the late 18th and early 19th centuries. Sir Walter Scott first visited in 1797 and became a regular visitor. He set part of his novel Meg Merrilees in the area. On one visit Scott met Charlotte Carpenter, his future wife, at the Spa Hotel and after a short time proposed by the river at the rock which became known as the 'Popping Stone' as it was here that he 'popped the question'. The rock is smaller now as the Victorians chipped off slivers of it as keepsakes."
Gilsland was not "briefly fashionable during the late 18th and early 19th centuries", it was galvanised by the opening of the railway in 1838. It must already have been popular before Scott visited in 1797, and judging by the amount of postcards sent from here in the early 20th century, it must have been "standing room only" up to WW1.
Walter Scott was not a regular visitor. He (famously) came here in 1797, then returned with his wife in 1805, a visit which was truncated by an alarm about a possible French invasion of Scotland. That's it.
Not one, but several of his novels are set here or draw on his experiences here - for instance Guy Mannering, which contains a description of Mumps' Ha', and St Ronan's Well, which draws on his experiences of the society at the Shaws hotel. Or how about "The Bridal of Triermain" ? OK - not a novel.
Meg Merrilees is a character from Guy Mannering, not a novel in her own right.
He met Ms Carpenter on the first visit.
He couldn't have met her at "the Spa Hotel" because there wasn't one at that time. The original burnt down in 1859 and was called Shaws in Scott's time.
According to his biographers, he probably stayed at Wardrew. Apparently he spotted Ms C while out riding, and "obtained an introduction" at a ball in The Shaws.
I can't discuss the rest without becoming apoplectic, see The Popping Stone for elucidation.