I haven't read much of it yet, though I know from reviews that it's a romance about a young woman whose affections are torn between a soldier (a miller's son driven to enlist by her flirtation) and his older dashing commander. But for the moment I'll just note that she's starting as she continued in later works, in terms of erudition, detailed landscape description, and precisely identifiable location. The opening paragraphs:
The Broken Tryst
Sweet Auburn! loveliest village of the plain,
Where health and plenty cheer'd the labouring swain,
Where smiling Spring its earliest visit paid,
And parting Summer's lingering blooms delay'd!
There is a little village on the south coast called Brightdale.
It is a sweet little quiet place, full of thatched cottages bosomed in luxurious myrtles and surrounded by fragrant old-fashioned gardens. It is sheltered on one side by breezy downs, over which the clouds float lazily, and the winds sweep freely; and on the other side the land slopes for a pleasant mile down to the sea, where it breaks off in loose cliffs of red sand, cut through by a deep gorge, worn by a small but rapid stream that turns a mill just below the village.
"Brightdale" is highly recognisable as Brighstone, a still-picturesque village near the south-west coast of the Isle of Wight (see also Google Maps); the gorge is Grange Chine. Coincidentally, it was Brighstone Library who a while back kindly sent me the biography and photo of MG from Charles John Arnell's 1933 Poets of the Wight.
The verses come from the beginning of Oliver Goldsmith's The Deserted Village. By coincidence, I was just looking at this article about possible inspirations for the poem: Angela Williams sent me a link to Abandoned Communities, Stephen Fisk's interesting (if somewhat depressing) site looking at depopulated locations in Britain (it includes literary and artistic responses). Of Devon interest: Hallsands is well-known.