Sometimes a painting's ostensible subject isn't its essential one, the artist having been forced (usually for financial reasons) to disguise his real motives, to keep his true self to the shadows. He has become, you could say, a stowaway in his own work. This, at least, seems to be the case with the many Gainsborough portraits that feature a far-off landscape of aqueous blues and greens, a feathery woodland that quivers in the distance. Sure, the dazzling silks of whichever countess or captain is striking a pose take centre stage, their surfaces revealing on close inspection the scuffed, luminous brushstrokes that predate late Monet by a century and a half. But there's something about that vague countryside that sticks in the memory, that makes it more than just a fashionably pastoral backdrop. Could it be that there always appears to be a storm brewing, the half-light pierced by gleams that beckon from the horizon? Or is it the way the blurred trees seem to have their own, swift velocity, seem to dance to a different drum? Whatever the reason for their allure, the backgrounds turn these society portraits into something else, into complex visual allegories concerning Nature and our place in it. Those dandies have their backs to the landscape - and yet you sense that at some point they're going to have to negotiate the dark, damp, restless woods, and that none of their finery is going to be of any help to them at all.

So, compelled to practise portraiture as a means of making a living ("the curs'd Face Business", he called it), Gainsborough managed to slip in glimpses of his own sombre Arcadia. His real love, though, was landscape painting per se, the creation of scenes that weren't so much copies of nature as imaginary equivalents of it. This is a commonplace of Gainsborough scholarship, but what Tate Britain's Gainsborough unintentionally establishes is that the landscapes are not only among the first by an English artist but also some of the greatest. To look at Sunset: Carthorses Drinking at a Stream (c1760) and A Grand Landscape (c1763), the two masterpieces of his early to mid-thirties, is to see the countryside recast as a vortex, a rippling whole in which man, beast, trees and sky are all conjoined. The brushwork has a fluency that animates everything it describes, the dead stump in the former work arching through the air like a strange wooden dragon, while the cloud-packed heavens have a physical presence equal to that of the land.

But it's essential to see these magnificent paintings in the flesh because reproductions give barely any sense of them at all. In fact, the power of Gainsborough's landscapes derives precisely from the fact that they take the fluid, whimsical style of the sketch or story illustration and elevate it to a monumental scale. These are big, imposing pictures that make no pretence of realism, that openly acknowledge themselves as fiction. They picture a world that both is and isn't our own, a highly stylised countryside with its own internal logic. So, much as you wouldn't normally think of mentioning Cezanne and Gainsborough in the same breath, they have a fair amount in common. Here, too, and despite Gainsborough's claims that he only included humanity in his landscapes "for the Eye to be drawn from the Trees in order to return to them with more glee", there's a strongly allegorical element. What are those distant, smoking, tree-embedded cottages that appear so often in his work if not embodiments of the perennial desire to live happily on the earth, to be enfolded in its arms? And yet, unified and remote, holding us back as much as they seem to draw us in, the landscapes are unenterable.

It's probably just as well, though, as we'd be sure to find the reality of mud, mulch and damp firewood far less palatable than our daydreams of it. Art does something other than provide a literal means of escape, and this is beautifully illustrated by one of Gainsborough's finest portraits. James Christie (1778) shows the eponymous auctioneer in a dark, sumptuous interior, the shining white of his cuffs and shirt-front resembling a dance of pure light. He leans elegantly on a gold-framed Gainsborough - not, as one might have expected, an easy-to-sell portrait but one of the largely unwanted landscapes. This painting-within-a-painting is even freer and rougher in execution than normal, a dense frenzy of cruel-looking trees that sits incongrously beneath Christie's manicured hand. Not a soul can be seen, just the forbidding wall of those fairytale woods. That outer world might be fundamentally fatal to us, but, as this portrait shows, a painting can bring its glow into the inner world of our homes.

Gainsborough is at Tate Britain, London SW1 (020 7887 8000) until 19 January 2003